Trying to understand DC/DC chargers
- Thread starter GSP
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DaveInDenver
Middle Income Semi-Redneck
It doesn't matter what loads you're running, the question will be how many batteries you intend to use, how you expect to charge them, where you locate them and the way they will be wired.
A DC-to-DC charger solves a specific problem you run into with batteries that are mounted some distance away or connected with smaller cables to the charging source. They do function to isolate batteries but if all you want is to separate house and starting batteries, particularly if they are close together or/and wired with heavy cable, then a relay or solenoid is all you need.
A DC-to-DC charger solves a specific problem you run into with batteries that are mounted some distance away or connected with smaller cables to the charging source. They do function to isolate batteries but if all you want is to separate house and starting batteries, particularly if they are close together or/and wired with heavy cable, then a relay or solenoid is all you need.
dwh
Tail-End Charlie
No, it's either DC-DC charger or isolator. If you have both, the isolator defeats the purpose of the DC-DC charger.
The DC-DC charger takes the voltage from the engine side and converts it to whatever voltage is best for charging on the aux side.
The isolator connects the engine side directly to the aux side, making the voltage the same on both sides.
Having both means the output from the aux side of the DC-DC charger just gets fed back around to its input on the engine side. A feedback loop that defeats the charger.
The DC-DC charger takes the voltage from the engine side and converts it to whatever voltage is best for charging on the aux side.
The isolator connects the engine side directly to the aux side, making the voltage the same on both sides.
Having both means the output from the aux side of the DC-DC charger just gets fed back around to its input on the engine side. A feedback loop that defeats the charger.
DaveInDenver
Middle Income Semi-Redneck
What he's said shouldn't be misunderstood to say a DC-to-DC charger shares the same negative of a diode isolator, which is a voltage drop. It can be thought of like a diode that has no drop and in fact will boost the voltage on the isolated side so the battery actually charges. Other than cost (DC-DC chargers can be pricey), the primary concern with them is they have a current limit.
DaveInDenver
Middle Income Semi-Redneck
I believe so, if I understand how the Smartpass works anyway. You'll leave one battery connected to the vehicle like normal and put the second one in the "Service" loop of the Smartpass. You'd want to run the alternator output into the D250S and it is controlling the charge on the house battery. Although I don't think in that circuit the solar panel will charge the main/vehicle/starting battery, only the service/house/aux battery.
DiploStrat
Expedition Leader
The SmartPass is an "intelligent" voltage sensing relay, like the Blue Sea ACR and others. The difference is that it integrates closely with the D250S.
The weakness of the D250S is that it will only produce 20A of charge. The SmartPass is an 80A relay that closes during the first part of the charge cycle, directly connecting your starter battery to the camper battery. This allows a greater charge rate in amperes. As the camper battery charges it will accept ever fewer amps. Once the rate drops to below 20A, the SmartPass opens and the S250S completes the charge, taking advantage of its boosted voltage.
While a B2B can compensate for voltage drop, you still have to wire it with appropriately large cabling.
See page 20, here: https://smartercharger.com/wp-content/uploads/2010/11/D250S-DUAL_EN.pdf
At the risk of being a bit of a shill, the REDARC BCDC 1240D (which I use on my truck which is a 24v alternator with a 12v camper battery) offers a 40A charge rate and several different profile to match your camper battery. It has a clever "green priority" circuit which means that if solar is available, it uses all of that first before pulling from the alternator. https://redarcelectronics.com/products/dual_input_40a_in-vehicle_dc_battery_charger
Worth noting that for many (most?) people, a B2B does not offer better performance. Most people will do better with an intelligent relay, the big exceptions being:
-- Low vehicle charging voltage, e.g. some Toyotas and Sprinters which do not sustain charging over 14.4v.
-- Cost savings due to integration of MPPT solar controller. (You don't have to buy a separate controller.)
-- Different battery types, e.g., AGM mixed with LiFePO4, where you want a different profile and may want to limit the load on the alternator.
Final note: All current B2B (CTEK, Sterling, REDARC) are one way chargers. If you want to use your camper battery to maintain your starter battery, you will need something like the AMP-L-START (http://www.lslproducts.net/ALS_Overview_Page.html) or a bypass relay or switch which you can engage manually as needed to charge or self-jump. (Or carry a Lithium jump pack.)
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Bear in NM
Adventurer
Diplostrat,
Thanks for chiming in. I was almost going to PM you with specific questions on the Redarc, as I saw that you were using a unit in another thread. I am exactly in your 3 exceptions. My new Ford e350 alternator only puts out 13.5-14 bouncing. I am looking very hard add a lifepo4 for a house bank, installed inside, directly over my starter batteries (lead acid) which are on the frame rail. And I have solar.
It sounds like you are happy with the Redarc. The only pause that I have is like to do components for things, to allow me to repair in the event something goes down. Are you happy with the charge controller that is built in to your unit? I need to do more research on the Redarc offerings, and figure out how much solar I want to put on my roof. And figure out where to buy.
Thanks,
Craig
Thanks for chiming in. I was almost going to PM you with specific questions on the Redarc, as I saw that you were using a unit in another thread. I am exactly in your 3 exceptions. My new Ford e350 alternator only puts out 13.5-14 bouncing. I am looking very hard add a lifepo4 for a house bank, installed inside, directly over my starter batteries (lead acid) which are on the frame rail. And I have solar.
It sounds like you are happy with the Redarc. The only pause that I have is like to do components for things, to allow me to repair in the event something goes down. Are you happy with the charge controller that is built in to your unit? I need to do more research on the Redarc offerings, and figure out how much solar I want to put on my roof. And figure out where to buy.
Thanks,
Craig
DiploStrat
Expedition Leader
I have been very happy with the REDARC BCDC 1240D. It puts out a solid 40A+, hour after hour. The small size and shock and water protection are nice features when compared to some of the competition. The base AGM profile has a slightly high voltage which assures that the batteries take all 40A.
I have not tried the LiFePO4 profile, but one of the nice features of a B2B when tying a LiFePO4 to an alternator set up for FLA or AGM is that the B2B will limit the amp draw and may thus protect the alternator from overload. Even a 100Ah LiFePO4 battery will suck a lot of amps - way more than an AGM, for example.
I have not tried the LiFePO4 profile, but one of the nice features of a B2B when tying a LiFePO4 to an alternator set up for FLA or AGM is that the B2B will limit the amp draw and may thus protect the alternator from overload. Even a 100Ah LiFePO4 battery will suck a lot of amps - way more than an AGM, for example.
Bear in NM
Adventurer
Thanks for the added details. I have only started my research on the LiFePO4, and need to dig a bit deeper on the brand that my solar shop carries. I am very concerned about my Arb 63 and whether the lowest voltage cut-off setting is going to limit me with a LiFePO4 battery. I am a lot weak on LiFePO4 SOC's, and am not sure whether I would be able to get the full 80% discharge. Kinda a big deal, for that kind of money. I am still a long way off from dropping the money on one of those, as I need to get panels first, But more firster is to find a basket for my roof cross racks, then I can figure what size panels I can swing. Trying to get the van correct on the first pass. For now I have my solar suitcase I can run inside, so that helps.
I definitely have a weak charging system, with a factory alt. trying to charge two group 65's in parallel to fire up my diesel. You addressed one of my concerns about taxing the alternator too much. I am clear that the LiFePO will suck the life out of any source, if given the chance. The high amp/fast recharge sure looks appealing. I will look more into the Redarc, but it sounds like you can set an amp ceiling on the unit? That would be handy.
Right now the plan is to get a line run from my dual starting batteries into the back of the van. The run will be short, just a few feet, but I am going to size the wires big, overkill for my solar suitcase, but enough to handle a LiFePO draw, if I go that way.
My only mppt controller currently is a Midnight Solar in my Cargo Trailer. It is a pretty big unit, with the fins and all. The other brand of pwm that I have are Sunsaver. Their MPPT is also large. The size of the Redarc really caught my eye. That's a lot of functionality in a small package.
Enough rambling.....
Thanks,
Craig
I definitely have a weak charging system, with a factory alt. trying to charge two group 65's in parallel to fire up my diesel. You addressed one of my concerns about taxing the alternator too much. I am clear that the LiFePO will suck the life out of any source, if given the chance. The high amp/fast recharge sure looks appealing. I will look more into the Redarc, but it sounds like you can set an amp ceiling on the unit? That would be handy.
Right now the plan is to get a line run from my dual starting batteries into the back of the van. The run will be short, just a few feet, but I am going to size the wires big, overkill for my solar suitcase, but enough to handle a LiFePO draw, if I go that way.
My only mppt controller currently is a Midnight Solar in my Cargo Trailer. It is a pretty big unit, with the fins and all. The other brand of pwm that I have are Sunsaver. Their MPPT is also large. The size of the Redarc really caught my eye. That's a lot of functionality in a small package.
Enough rambling.....
Thanks,
Craig
DaveInDenver
Middle Income Semi-Redneck
You have to follow best practices for safety for the device's current but voltage drop is no longer a primary concern.
If you choose a 40A DC-to-DC charger and your run is perhaps 20 feet you could get away with 8 AWG using 105 C insulation. In a fully passive cabling a 40 A, 20 foot run like that might dictate 1AWG at least, which would get voltage drop down to 0.1 V.
If select an 80 A DC-to-DC charger you can wire that with 4 AWG safely while this would probably require 2/0 AWG to get a usefully low voltage drop normally.
The real benefit is that none of it matters, as long as you don't undersize so much that you melt the insulation on your cables the battery will get a correctly profiled charge.
DiploStrat
Expedition Leader
You don't set a limit; the B2B can only put out what it is is designed to produce. So in the case of the 1240, it is fused at 60A (max draw) and produces about 40A, max output. The difference being the loss to heat, voltage boost, etc.
Bear in NM
Adventurer
DiploStrat,
Thanks for the clarification. 40-60 amps is not into winching territory, but certainly like running aux lighting (older style) or such. Not excessive, but not insignificant.
Dave, in my case I have 5-6 feet of run. Pretty short.
Craig
Thanks for the clarification. 40-60 amps is not into winching territory, but certainly like running aux lighting (older style) or such. Not excessive, but not insignificant.
Dave, in my case I have 5-6 feet of run. Pretty short.
Craig
DiploStrat
Expedition Leader
The trick is that while winches draw a lot of amps, they don't do it for long. A LiFePO4 battery can pull hundreds of amps for hours - possibly a formula for a fried alternator. As a B2B limits the current that it can provide, it may be a safer alternative. In my direct connect tests we got some cables VERY hot!