How to trickle charge starter battery from house battery set up?

[Superduty]

Any thoughts or suggestions on utilizing the house battery system to trickle charge the starting batteries if the truck sits for an extended period? Hopefully there is a reasonably priced solution.

I found this item, AMT I am not sure if it will work with the Victron DC to DC.


Current set up on a diesel truck with camper shell is the following.

Solar panel
100 amp hour LifePo4 (house battery)
Victron smart solar MPPT
Victron DC to DC charger from truck to house battery
Two FLA starting batteries



Please do NOT suggest a shore power set up.

 

[Ozarker]

I have a bluetti set up so I'm not familiar with your gear, bur you're saying you have a DC charger from truck to house.

Can't you use the solar to the truck battery first then "overflow" to the house battery? Reverse the connection from the DC charger?

 

[Superduty]

I have a bluetti set up so I'm not familiar with your gear, bur you're saying you have a DC charger from truck to house.

Can't you use the solar to the truck battery first then "overflow" to the house battery? Reverse the connection from the DC charger?

I suppose the DC to DC could be reversed but I think that would entail switching the IN/OUT wires each time on the Victron DC to DC.

 

[DiploStrat]

With a pair of lead acid batteries, the simple answer used to be an intelligent relay, like the Blue Sea ACR.

With a lithium camper battery and a lead acid starter, the economic answer used to be the AMP-L-Start, but they may have gone out of business. There seem to be some still in stock.

The expensive way, which I use for various reasons, is a pair of B2B (or DC-DC chargers), with an ignition controlled relay to interrupt the circuit. Works very well.

 

[Superduty]

With a pair of lead acid batteries, the simple answer used to be an intelligent relay, like the Blue Sea ACR.

With a lithium camper battery and a lead acid starter, the economic answer used to be the AMP-L-Start, but they may have gone out of business. There seem to be some still in stock.

The expensive way, which I use for various reasons, is a pair of B2B (or DC-DC chargers), with an ignition controlled relay to interrupt the circuit. Works very well.

I was thinking of using a Blue Sea ACR (I already have one). I could set it up with an Anderson type plug and just plug it in when needed. What might be the drawback of doing this?

I will look at the Amp-L-start. Looks like the man that made these passed away in January 2023 and it is unknown if they will continue.

I agree the B2B set up is probably a good way to go, but that would entail buying another DC to DC unit...probably more than I want so spend.

 

[dstefan]

Why not just use your solar with a cheap 2nd controller? I do this occasionally with a cheap PWM controller from my portable panel.

 

[Ozarker]

I suppose the DC to DC could be reversed but I think that would entail switching the IN/OUT wires each time on the Victron DC to DC.

Switching could elevate reversing the wiring, just flip the switches.

 

[Superduty]

@DiploStrat I edited my original post to add a reference to this item. Any thoughts?

Why not just use your solar with a cheap 2nd controller? I do this occasionally with a cheap PWM controller from my portable panel.

I am not sure what a PWM controller is? Recommendations for a cheap secondary controller? There are some very cheap ones on Amazon such as THIS ONE. I didn't realize they are available so cheap. But I am not sure these cheap ones will work my solar panel - see photo.

 

[dstefan]

PWM is Pulse Width Modulated. It's a less efficient approach than the MPPT controllers in general.

Yeah, you may exceed the wattage/voltage for the cheapos.

FWIW -- I mentioned it because I used to use a 120 watt portable panel from Acopower that came with a PWM controller when I had my old rig with dual AGM batteries under the hood. It was OK for keeping them up when stationary, but rarely pushed more than 3.5 amps. That was partly due to the PWM controller and partly due to them putting the controller about a foot from the panel and then running a 15 foot 14 awg cable to a set of jumper clamps. Not ideal!

When I got the camper, I put in a National Luna portable power pack with an integrated DCDC charger from the alternator and an integrated NL MPPT controller with a 100ah Battleborn. So, I cut off the 14 gauge and PWM controller on the panel and replaced with 20 ft of 10 gauge PV cable feeding directly into the NLPP's MPPT controller. Started yielding 6 amps in full sun!

I then repurposed the PWM controller so I could plug in the panel feed from the good 10 gauge PV cable into the old PWM controller with a short throw (about a foot) to battery clamps as an emergency starter battery charger/trickler. Never had to use it, other than to test it, but it works fine. It pushes more amps even with the PWM controller as the wire gauge is now appropriate and the controller is close to the battery.

Maybe a relatively cheap portable kit would work for you? Kinda overlaps with all that wattage you have, though. There are also much more affordable MPPT controllers too, and maybe one of those could handle your panel specs.

 

[Superduty]

Fwiw,
I use that same controller along with a 50W solarmodule to keep my tractor charged. It works great.
Simply to keep trickle charge on an idle battery, its hardly worth the extra cost of MPPT.

The cheap solar controller seems like a good solution for me. I am just not sure whether it can handle my solar panel?

 

[DiploStrat]

Seems that the owner of AMP-L-Start passed away and, as it was basically a one person shop, the company is tied up in estate issues. Pity. If you can find the lithium model buy it.

Beyond that, double tapping one set of solar panels sounds interesting, but I have never done it, so I cannot comment on the pros and cons.

The basic Blue Sea ACR won't work for a heterogeneous lead acid/lithium set up as the voltages are wrong. Most intelligent relays close at around 13.2v, because a lead acid battery will never reach that voltage unless under charge, and open at 12.7v, nominal "full" charge for lead acid. Most lithiums tend to rest at between 13.3v and 13.4v, so the ACR will never open.

Victron sells an intelligent relay that they claim has proper settings for a mixed lead acid/lithium combo.

I use a second B2B which is wired with an ignition controlled relay so that it is switched "off" when the engine is running. The B2B has conventional lead acid voltages, but that has not proved to be an issue as my camper battery is large (>800Ah) and the starter battery quickly gets to float and thus the drain is minimal.

Looking at the Sprinter and other fora, there seems to be some chatter on the search for alternatives to the AMP-L-Start - Google may be your friend.

 

[DiploStrat]

The cheap solar controller seems like a good solution for me. I am just not sure whether it can handle my solar panel?

PWM units generally need to be held to less than 32v, so, if your panels are in parallel (like mine) they are likely to be running at over 50v.

I like the idea of double tapping the panels to a second controller, I just don't know the implications of doing this.

For the task of simply maintaining a lead acid starter battery you do not need a super sophisticated charger. (One reason why the AMP-L-Start was such a great product.

 

[jonyjoe101]

On my astrovan after several years of the battery getting drained over the weekend and slow cranking after sitting for several days I finally checked what the drain was. Hooking up a DC amp meter (start battery cable) to see how much power the van was using with engine off and everything turned off, I was shocked to see the van used 290ma an hour (as per sources on the internet it shouldnt be higher than 30ma). I still havent track down what is causing the drain.
I have been using a boost/buck converter to keep the start battery topped off from my 220ah lifepo4 through the cigarette plug, I added an overvoltage control relay so its basically automatic (it charges to 14.4 volts then shutoff and when the battery drops to 12.3 volts it restarts the cycle). I had to build my own since I wanted an LED display to view all battery voltages in realtime.
This has helped tremendously, it uses maybe 6 amps every 2 days to keep the start battery at full charge.
Without any DC to DC charger I would have to disconnect the start battery cables whenever it sat more than 24 hours. I have a 365 watt panel always charging the lifepo4 so the 6 amps the DC charger uses every 2 days is insignificant.
The AMT12-2 unless its programmable the cut-in voltage (to charge the start battery) is too low at 11.5 volts (which is below 50 percent for lead acid) its doubtful if the battery will crank the engine if it gets that low. My DC charger I have it set to 12.3 volts to cut in.
Also the cutoff on the AMT12-2 for the house battery 11.7 volts is too low for lead acid or lifepo4. It should be about 12.1 volt for lead acid and 12.8 for lifepo4.

 

[Superduty]

@jonyjoe101 I originally read the AMT12 specs about 11.5 and 11.7 as you did. it made no sense. After re reading it a few times, I think the 11.5 and 11.7 are the low voltage levels where the system shuts off. It actually charges the start battery to 12.8v

 

[burleyman]

Any thoughts or suggestions on utilizing the house battery system to trickle charge the starting batteries if the truck sits for an extended period? Hopefully there is a reasonably priced solution.

Current set up on a diesel truck with camper shell is the following.

Solar panel
100 amp hour LifePo4 (house battery)

Two FLA starting batteries

I have been using boost circuit boards for years to do that. Boost circuits only increase voltage, which is what is needed when a higher voltage than resting battery voltage is required to charge another battery.

Locate the boost module close to the start battery(s), same as with a solar charge controller.You simply connect the boost module's input to the source battery with long small wires (16-18 awg), and the output wires to the battery being charged. Dial in the charging voltage required for a trickle charge, then walk away.

When arriving at camp, both batteries should be charged, so initial charge current will be low.

Fuse the input for 5 to 10 amps. If you don't disconnect the module before cranking, the lowered starting battery voltage can cause a high module output, and fry it. Learned the hard, but inexpensive way.

I use the same module to keep the voltage level up to my 12v fridge to avoid low voltage beeping, blinking, and shutdown.