DC-DC charger as an alternative to generator?
- Thread starter Martinjmpr
- Start date
Nope, running it from the trucks lead acid battery to a victron dc-dc charger set to lithium to my 2, 12volt LiFePo4 batteries. It has been working flawlessly for 1.5 years. I believe this is what the OP was asking about in his original post.
Last edited:
Dave in AZ
Well-known member
I'm gonna say misinformation here. I subscribe to Clark's channel and watched all his vids. He is a boat guy.
First, His video is all centered around small, weak alternators on boat engines, that ABSOLUTELY HAVE to keep working. With large electrical loads. NOT on a standard truck trying to use 20 to 40A from a truck alternator that provides 130 to 260A, and a truck that uses 30 to 50A, leaving 40 to 100A extra capacity while STILL remaining under 60% loads.
Second, no one would use a lead acid battery today as a truck house battery, who has done even 1 hr of research.
Third, Lithium batteries absolutely do NOT damage or hurt alternator, IF YOU USE A DCDC charger, that is precisely what they are designed to prevent, and is the driving reason anyone uses one!
Fourth, that victron video has been debunked as bogus rpm and loads. I do however still sometimes link it to folks myself... but its whole point is that you need to stay below 70% rated max or even lower, when using a consistent load, to keep from overheating alternator and burning it. (This is why that vid has been debunked often, they didn't run the alt at standard actual rpm seem from a reducer pully, just ENGINE IDLE RPM, most pullies are 10x rpm increase for alternators. This would give massively more airflow for cooling from built in fan. Also some alternator size differences and obvious choices made to "force a failure for the video", that weren't made clear to viewers.)
Last edited:
So all DIY builders do this?
Dave in AZ
Well-known member
I would say that most folks who are running a 2nd battery i.e. "House battery" system for overlanding, DO indeed run a dc to dc charger from their alternator. Back 15 years ago, folks used a solenoid battery isolator to separate their start battery from house system, but with the virtually 100% advent of "smart alternators" since 2005, a dc to dc charger is now required. It is needed separate the two systems; protect alternator and car electronics from Lithium battery high amp stoppage when a lithium BMS shuts battery off; keep starter battery from being drained; and provide correct outgoing voltage to properly charge lithium OR lead acid batteries.
Usual charging amperage for 12V systems are 20, 25, 30. A few 50A do exist, but can also be set lower. Most contain instructions to avoid going over 70% alternator rating.
Additionally, a specialized dc2dc charger that ALSO boosts output voltage to 42 to 60V have flourished this year, to allow fast charging through the solar input ports in PowerStations i.e. "solar generator PowerStations ".
I would say that these are basically removing the need or desire for solar panels in van and truck sized setups, as you can get the same Watts as a completely covered roof, 500W, but don't have to worry about sun, much more reliable.
I run mine, a "Pecron 500W Car charger", and have virtually stopped carrying my bulky 460W of portable solar panels. In winter if sitting still all day, I can maybe get 200W output for 5 hours, 1000 Watt-hrs a day. But I can get that ALWAYS no matter the weather or time, in 2 hours driving around with dc2dc. That adds 37A to my alternator, which on a sunny good weather day totals 59A load, easy for my 130A Tacoma alternator. When my alternator has a lot of loads, like night with rain, I dont charge with dc2dc, or drop it to 20A only to stay at 60% alternator loading max.
There is a whole subforum on DIYSolarForums, Vehicle Mounted, with zillions of posts and system diagrams etc., the best resource online for mobile solar etc
Vehicle Mounted Systems
RV's, Vans, Trucks and More! Typically 12v, and 100-1000w, but the limits are endless!
diysolarforum.com
I have a bunch of videos showing alternator load testing, how much each item on a truck adds to load (headlights, hibeams, turn signal, wipers, radio, airco, heater, etc). Then also various mobile truck PV setups showing power delivered, winter vs summer, etc. Then a few on dc2dc setups, proper cable sizing, alternator loading. Then a series on electric cooking requirements, like coffee, full breakfast, meal reheats like stew, omelets, etc. Hmmm, then a few on how to setup a correct system with a dc fuseblock, protecting and fusing circuits, daily overlandlanding power usage, diesel heater power usage, dc fridge power usage, LEDs and fans, heating blankets.
Here is a link if you're interested in browsing some, I try to be fast pure info, zero BS, not monetized at all.
I would say that most folks who are running a 2nd battery i.e. "House battery" system for overlanding, DO indeed run a dc to dc charger from their alternator. Back 15 years ago, folks used a solenoid battery isolator to separate their start battery from house system, but with the virtually 100% advent of "smart alternators" since 2005, a dc to dc charger is now required. It is needed separate the two systems; protect alternator and car electronics from Lithium battery high amp stoppage when a lithium BMS shuts battery off; keep starter battery from being drained; and provide correct outgoing voltage to properly charge lithium OR lead acid batteries.
Usual charging amperage for 12V systems are 20, 25, 30. A few 50A do exist, but can also be set lower. Most contain instructions to avoid going over 70% alternator rating.
Additionally, a specialized dc2dc charger that ALSO boosts output voltage to 42 to 60V have flourished this year, to allow fast charging through the solar input ports in PowerStations i.e. "solar generator PowerStations ".
I would say that these are basically removing the need or desire for solar panels in van and truck sized setups, as you can get the same Watts as a completely covered roof, 500W, but don't have to worry about sun, much more reliable.
I run mine, a "Pecron 500W Car charger", and have virtually stopped carrying my bulky 460W of portable solar panels. In winter if sitting still all day, I can maybe get 200W output for 5 hours, 1000 Watt-hrs a day. But I can get that ALWAYS no matter the weather or time, in 2 hours driving around with dc2dc. That adds 37A to my alternator, which on a sunny good weather day totals 59A load, easy for my 130A Tacoma alternator. When my alternator has a lot of loads, like night with rain, I dont charge with dc2dc, or drop it to 20A only to stay at 60% alternator loading max.
There is a whole subforum on DIYSolarForums, Vehicle Mounted, with zillions of posts and system diagrams etc., the best resource online for mobile solar etc
Vehicle Mounted Systems
RV's, Vans, Trucks and More! Typically 12v, and 100-1000w, but the limits are endless!
I have a bunch of videos showing alternator load testing, how much each item on a truck adds to load (headlights, hibeams, turn signal, wipers, radio, airco, heater, etc). Then also various mobile truck PV setups showing power delivered, winter vs summer, etc. Then a few on dc2dc setups, proper cable sizing, alternator loading. Then a series on electric cooking requirements, like coffee, full breakfast, meal reheats like stew, omelets, etc. Hmmm, then a few on how to setup a correct system with a dc fuseblock, protecting and fusing circuits, daily overlandlanding power usage, diesel heater power usage, dc fridge power usage, LEDs and fans, heating blankets.
Here is a link if you're interested in browsing some, I try to be fast pure info, zero BS, not monetized at all.
Watched several youtube videos whereas folks drive 2-4hrs a day just to keep the batteries charged, and when they have too few solar panels.
I'd hate to leave a beautiful campsite two days early just to drive my batteries to a charged state.
I added to this thread these setups are too expensive.
Added electronics, dc to dc charger - wires and heat monitoring the alternator - house battery, maybe 2nd alternator.
Costing $400
A nice quite generator, say 3600watts is $390 without all the work. And the charge volts is much higher through a automatic transfer switch.
Last edited:
Just please don't set up camp next to me.
Doing DC-DC charging properly, you only drive to charge it while on the move, that is, from place to place or if you need to re-stock on supplies. Going for a dedicated drive would be about the most stupid thing one could do as it defeats the whole point of getting out in the wilderness. Obviously dc-dc chargers or vehicle charging are only valid if your constantly on the move every few days and fail hard when staying in one spot for a long time. And Generators are not allowed in many places. This is where large roofs with highest efficiency panels or portable panels and ultra efficient loads start to become a necessity. My 125L fridge draws average 2A per hour in 40degree heat with the dial set to 1degree C, my LED lights draw about 1W max, my 16inch workstation laptop draws about 10-15W max for heavy multitasking.
Get the panels and loads as efficient as possible and you will be happier, I cut off anything i dont need, like freezers, nonsense induction cookers, inverters etc.
Martinjmpr
Wiffleball Batter
Warning: Long post:
OK, I went 'dark' for a few days there when I didn't have reliable internet access and/or was on the road and driving and couldn't respond. We got back to Pueblo on 12/11 and since then I've been catching up on household stuff. I finally have enough time to dive back into this.
My batteries ( 2 x 100AH lithium) both went to zero on about day 3.5 of our trip despite my attempt to charge them. We had last hooked up to electric power on Sunday morning (11/24.) Drove from Pensacola to Ocala NF (presumably charging via both the 7 pin on the trailer and also via the 100AH panel on the roof.) I didn't check the power level on Sunday night when we camped at Ocala.
We got into Key West after driving all day and arrived around 5:00 PM on Monday 11/25. By Tuesday night, 11/26, 8:00pm my battery level was at 67% as measured on the batteries own app. I had attempted to charge using an 8 AWG cable and 2 large anderson connectors running from the Renogy 20A DC-DC charger in my truck (that I use to keep the battery for the truck fridge charged up.)
Note the current shown: 3.12A. This is when hooked to what is supposed to be a 20A charger.
By 9:30pm on Wednesday 11/27 battery levels were down to 32%. I should point out that it was warm (70 - 78) and sunny in Key West. I did have a solar panel but on the curved surface our our travel trailer the panel never actually faced the sun and never put out more than 1.5A - 1.8A (as shown on the control panel of the solar controller.) Apparently not enough to keep up with the fridge. I again attempted to charge the battery using a cable from the DC-DC charger and while it did show charging, the charge was never more than about 3 - 4A.
QUESTION: Am I right in thinking that the Current display on the battery shows NET amps? In other words, what I'm asking is, if the charger is putting in, let's say, 6 amps, but the fridge (12v compressor fridge) and maybe the lights are DRAWING 4 amps, the display is going to show + 2A.
I did notice that with no external power connected, sometimes the current would show 0 and sometimes it would show -4.5 or -2.5. I'm thinking this would have been when the compressor was running on the fridge.
By Thursday evening, 11/28 (Thanksgiving Day), power was down to 8%. Again I attempted to charge with the DC-DC charger:
While it did show charging, it was barely charging and nowhere near 20A.
Batteries died that night. On Friday morning, I pulled them out and charged them individually by carrying them over to the laundry room and plugging in a 120v AC charger that I had brought with me.
Fun Fact! I discovered that when lithium batteries discharge they "go to sleep" and simply connecting them to a charging source won't "wake them up" so they can be charged. I ended up having to use jumper cables from my my portable (lead-acid) battery to get them to wake up so I could then charge them. Charged each battery to about 33% (which took around an hour for each one) and then put them back in.
By the evening of 11/29 the batteries were down below 10% again so rather than have to run through the step of "waking them up" I simply disconnected them so they wouldn't go to zero. We were departing the morning of 11/30 so I re-connected them before leaving, knowing that they would be charged by the 7 pin connector and by the solar panel as we drove, and in any case we would have electric hookups the rest of the time we were camping so power was a non-issue.
(To be continued in the next post.)
OK, I went 'dark' for a few days there when I didn't have reliable internet access and/or was on the road and driving and couldn't respond. We got back to Pueblo on 12/11 and since then I've been catching up on household stuff. I finally have enough time to dive back into this.
My batteries ( 2 x 100AH lithium) both went to zero on about day 3.5 of our trip despite my attempt to charge them. We had last hooked up to electric power on Sunday morning (11/24.) Drove from Pensacola to Ocala NF (presumably charging via both the 7 pin on the trailer and also via the 100AH panel on the roof.) I didn't check the power level on Sunday night when we camped at Ocala.
We got into Key West after driving all day and arrived around 5:00 PM on Monday 11/25. By Tuesday night, 11/26, 8:00pm my battery level was at 67% as measured on the batteries own app. I had attempted to charge using an 8 AWG cable and 2 large anderson connectors running from the Renogy 20A DC-DC charger in my truck (that I use to keep the battery for the truck fridge charged up.)
Note the current shown: 3.12A. This is when hooked to what is supposed to be a 20A charger.
By 9:30pm on Wednesday 11/27 battery levels were down to 32%. I should point out that it was warm (70 - 78) and sunny in Key West. I did have a solar panel but on the curved surface our our travel trailer the panel never actually faced the sun and never put out more than 1.5A - 1.8A (as shown on the control panel of the solar controller.) Apparently not enough to keep up with the fridge. I again attempted to charge the battery using a cable from the DC-DC charger and while it did show charging, the charge was never more than about 3 - 4A.
QUESTION: Am I right in thinking that the Current display on the battery shows NET amps? In other words, what I'm asking is, if the charger is putting in, let's say, 6 amps, but the fridge (12v compressor fridge) and maybe the lights are DRAWING 4 amps, the display is going to show + 2A.
I did notice that with no external power connected, sometimes the current would show 0 and sometimes it would show -4.5 or -2.5. I'm thinking this would have been when the compressor was running on the fridge.
By Thursday evening, 11/28 (Thanksgiving Day), power was down to 8%. Again I attempted to charge with the DC-DC charger:
While it did show charging, it was barely charging and nowhere near 20A.
Batteries died that night. On Friday morning, I pulled them out and charged them individually by carrying them over to the laundry room and plugging in a 120v AC charger that I had brought with me.
Fun Fact! I discovered that when lithium batteries discharge they "go to sleep" and simply connecting them to a charging source won't "wake them up" so they can be charged. I ended up having to use jumper cables from my my portable (lead-acid) battery to get them to wake up so I could then charge them. Charged each battery to about 33% (which took around an hour for each one) and then put them back in.
By the evening of 11/29 the batteries were down below 10% again so rather than have to run through the step of "waking them up" I simply disconnected them so they wouldn't go to zero. We were departing the morning of 11/30 so I re-connected them before leaving, knowing that they would be charged by the 7 pin connector and by the solar panel as we drove, and in any case we would have electric hookups the rest of the time we were camping so power was a non-issue.
(To be continued in the next post.)
Last edited:
Martinjmpr
Wiffleball Batter
(Continued from previous post)
The short answer regarding my question is that, at least as I'm currently set up, the DC-DC charger is NOT suitable for keeping the lithium batteries charged. Accordingly, we are shopping for a small generator (probably ~2000w), quiet and preferably dual fuel (gasoline/propane) so we don't have to carry gasoline (we already carry propane, dual tanks on the trailer.)
But the big question to me is WHY is the Renogy DC-DC charger only putting out a paltry 5 - 8 A when charging? I've never had a way to measure the current output before but it's supposed to be a 20A charger. It is connected directly to the battery using 8AWG cable straight to the battery terminals - I'm not sure of the exact length but it is probably around 10' and certainly no more than 15'.
According to this wire size calculator: https://www.wirebarn.com/Wire-Size-Calculator-_ep_41.html
8 AWG should be adequate with a 2% voltage drop from the battery to the DC-DC charger (in fact, that calculator shows that 20A @ 13.8v should be good up to 21 feet.)
Even if you measure both the positive and negative cables and combine them, worst case it's 30' which, again according to that chart, should be good for less than a 5% voltage drop. Doing the calculations that should mean roughly 13.11v. Is that too low to charge?
I guess the simplest thing could be: Is the DC-DC charger just defective? What would be the best way to test the actual output?
At this point I'm resigned to getting a generator but I'd still like to figure out why my 20A DC-DC charger appears to be putting out a fraction of its rated current.
Any thoughts would be welcome!
EDITED TO ADD: Further troubleshooting of the DC-DC charger will be done. I'll post the results. I want to measure the voltage at the battery and then where the power goes IN to the DC-DC charger to see if maybe the issue is excessive voltage drop.
UPDATE: No significant voltage drop measurable. 14.5 at the battery with the engine running and 14.44 showing at the point where the 8AWG cables go into the DC-DC charger. So the charger itself seems to be getting power. Is there such a thing as current drop over the length of a cable and how would I measure that? Clamp-on ammeter?
The short answer regarding my question is that, at least as I'm currently set up, the DC-DC charger is NOT suitable for keeping the lithium batteries charged. Accordingly, we are shopping for a small generator (probably ~2000w), quiet and preferably dual fuel (gasoline/propane) so we don't have to carry gasoline (we already carry propane, dual tanks on the trailer.)
But the big question to me is WHY is the Renogy DC-DC charger only putting out a paltry 5 - 8 A when charging? I've never had a way to measure the current output before but it's supposed to be a 20A charger. It is connected directly to the battery using 8AWG cable straight to the battery terminals - I'm not sure of the exact length but it is probably around 10' and certainly no more than 15'.
According to this wire size calculator: https://www.wirebarn.com/Wire-Size-Calculator-_ep_41.html
8 AWG should be adequate with a 2% voltage drop from the battery to the DC-DC charger (in fact, that calculator shows that 20A @ 13.8v should be good up to 21 feet.)
Even if you measure both the positive and negative cables and combine them, worst case it's 30' which, again according to that chart, should be good for less than a 5% voltage drop. Doing the calculations that should mean roughly 13.11v. Is that too low to charge?
I guess the simplest thing could be: Is the DC-DC charger just defective? What would be the best way to test the actual output?
At this point I'm resigned to getting a generator but I'd still like to figure out why my 20A DC-DC charger appears to be putting out a fraction of its rated current.
Any thoughts would be welcome!
EDITED TO ADD: Further troubleshooting of the DC-DC charger will be done. I'll post the results. I want to measure the voltage at the battery and then where the power goes IN to the DC-DC charger to see if maybe the issue is excessive voltage drop.
UPDATE: No significant voltage drop measurable. 14.5 at the battery with the engine running and 14.44 showing at the point where the 8AWG cables go into the DC-DC charger. So the charger itself seems to be getting power. Is there such a thing as current drop over the length of a cable and how would I measure that? Clamp-on ammeter?
Last edited:
Dave in AZ
Well-known member
I think on 1st glance that everything is working correctly, and you have just failed to abide by required engineering i.e. cable sizes.
1. For DC circuit cable voltage drop calcs, THE ENTIRE CIRCUIT LENGTH IS USED! POSITIVE AND NEGATIVE BOTH!
2. Length matters...someone should have told you this by now
Why are you guessing cable length? Use a tape measure.15ft x 2 is 30ft.
With 14V, 30ft 8awg, flowing 20A, there is a 6% voltage drop. That leaves you with 13.1V, which is 40% charge on an LFP. IF you were directly connected, no dcdc, you'd reach 40% charge and then amps would taper off to zero.
3. Voltage drop measurements depend on amps flowing. At 0.1A, basically no current flow, just testing the end of your 8awg cable, there is a 0.03% voltage drop. At 20A there is 6%. So if you tested that 14.45v at end of cable with no current, that doesnt mean anything. Why don't you measure the voltage at the house battery while it's all hooked up and charging?
4. This depends on the specs of that 20A charger. What is it's required input voltage? If it is low, can it still output 20A at 14.0v? Or does it drop the output voltage in relationship to the input dropping? If so, here is an example: 13.0v output because input dropped to 13.1v due to cable drop. Thats like 30% LFP charge. Say your LFP is AT 29% charge, you'll see low amps gradually going to zero as battery reaches 13.0V and 30% charge.
I don't know how the renogy 20A works, and they don't have it on website, and I'm not gonna read the owners manual. But YOU should
I thought I sent you links before to a video showing what cable size is needed for dcdc charger, and the answer was 6awg, not 8awg. Here it is again for you:
Dave in AZ
Well-known member
@Martinjmpr
Meh I downloaded the manual for the OLD 1212 20.
Here are the potential mistakes you may have made and issues I see:
1. Did you wire an ignition source to the D+ terminal? If not, charger won't work.
2. Did you make a mistake and wire into the LC terminal instead? Instant derate of unit to 10A max.
3. Did you set all the DIP switches correctly for LFP and proper voltages?
There are 5 of them that must be all set per their table to adjust output voltage from 12.6 to 14.6 volts.
Here is what I'd pick:
Dip sw5 OFF for lithium, on is for lead acid
I would use 14.2V as the Absorption or "bulk" charge rate. You could use 14.4V for faster, but 14.6V can cause cells to overvoltage and the BMS will disconnect the battery, dont use 14.6v.
Here are the settings for 14.2V:
Sw1 on
Sw2 off
Sw3 on
Sw4 on
Sw5 off
Let me know what you had set.
Also, look at the LED fault light on the 1212 20 dcdc unit, is it on? There is a troubleshooting guide on pg 20 of manual for that...
P.s. owners manual pg 9:
21 to 30ft circuit, starter batt to dcdc cable 6 or 4awg; dcdc to house batt 6awg.
"
When the battery charger is sending the rated amps, the input side
may experience a draw higher current draw by a factor of up to 50%. Larger wire sizes general-
ly improve performance, whereas smaller wire sizes may reduce performance, especially if
undersized. "
Meh I downloaded the manual for the OLD 1212 20.
Here are the potential mistakes you may have made and issues I see:
1. Did you wire an ignition source to the D+ terminal? If not, charger won't work.
2. Did you make a mistake and wire into the LC terminal instead? Instant derate of unit to 10A max.
3. Did you set all the DIP switches correctly for LFP and proper voltages?
There are 5 of them that must be all set per their table to adjust output voltage from 12.6 to 14.6 volts.
Here is what I'd pick:
Dip sw5 OFF for lithium, on is for lead acid
I would use 14.2V as the Absorption or "bulk" charge rate. You could use 14.4V for faster, but 14.6V can cause cells to overvoltage and the BMS will disconnect the battery, dont use 14.6v.
Here are the settings for 14.2V:
Sw1 on
Sw2 off
Sw3 on
Sw4 on
Sw5 off
Let me know what you had set.
Also, look at the LED fault light on the 1212 20 dcdc unit, is it on? There is a troubleshooting guide on pg 20 of manual for that...
P.s. owners manual pg 9:
21 to 30ft circuit, starter batt to dcdc cable 6 or 4awg; dcdc to house batt 6awg.
"
When the battery charger is sending the rated amps, the input side
may experience a draw higher current draw by a factor of up to 50%. Larger wire sizes general-
ly improve performance, whereas smaller wire sizes may reduce performance, especially if
undersized. "
Martinjmpr
Wiffleball Batter
Martinjmpr
Wiffleball Batter
So after all this it seems I'll need to go to 6AWG cable in order to get the 20A charger to be able to put out max output. I presume something like this would work?
I like this one because it has double sheathing and the pos and neg cables are not separate, which should make it easier to install. I'm guessing I'll have to drill a bigger hole in the cab than the one I have for the 8AWG cable that's in there now, but at least now I have a likely culprit for my issue. Thanks for the help!
I like this one because it has double sheathing and the pos and neg cables are not separate, which should make it easier to install. I'm guessing I'll have to drill a bigger hole in the cab than the one I have for the 8AWG cable that's in there now, but at least now I have a likely culprit for my issue. Thanks for the help!
Dave in AZ
Well-known member
No prob. However... Why don't you just go 4Awg, and be ready for your eventual 30A or 50A charger? Totally worth the one time cost to do it correct once.
Also, you're still using those oooold FLA batteries! We get less than 2 years here in Phoenix from a car FLA battery before it doesn't charge right! I thought you had replaced with LFP ones. When you eventually DO, you're gonna want a 50A charger, so just get the 4AWG cable now, even the 20A is on the edge of needing 4awg at 30ft circuit already.
Between old FLA batteries and 8awg, I can see your charge issues easily.
P.s. I just drove up and down road today, testing output watts and amperage draws on my alternator! For a Pecron 500W dcdc car charger, that boosts output to 42V for use with powerstations. Took a lot of video, editing it. However, info for you:
Tacoma, with full electrical load I'd expect at night in rain (hibeams, wipers, full air-conditioning, turn signal, brakes applied for powerbrakes and tail lights), gave 57A! Brakes bump it up a lot, 21A momentarily.
With 8.2A to run car with no loads,
20A dcdc output, 268W, required 24A input on alternator to achieve. When I simulated this with car fully liaded up, like night in rain with wipers, signalling a turn, I got 64A load on alternator. Applying brakes briefly gave 75A loads for 5 to 10 seconds. Once driving straight, I was 63A load again.
Same setup but no car loads, so a sunny day with lights off, I was at 32.5A steady alternator load. Just to help you guesstimate your supply and demand ability