Charging a LiFePO4 battery in parallel with a FLA starting battery.

[packrat1969]

Can anybody recommend in an inexpensive automotive-style charger that can isolate and replenish a LiFePO4 battery from a car's alternator (at the proper voltage levels) while the car is running, and then, when the engine is shut off, connect the LiFePO4 battery in parallel with the starting battery. This would be done to bolster the FLA starting battery's accessory runtime power, but not subject the LiFePO4 battery to the high starting current that the FLA battery sees.

packrat

 

[chicagoandy]

I'm not sure of a product that does exactly that, but a B2B charger like these Sterling ones would get you close:

Sterling Power battery to battery charging system - 12v 40 amp DC powered charger- BB1240

Part # BB1240 Now Available. Our Newest DC to DC charger in a 12v 40amp version.

www.sterling-power-usa.com

It will:
1. Properly charge the LFP Battery from the alternator, with the correct charging profile for LFP.
2. When you start pulling on your starting battery with an accessory, it will allow current to flow back to the starting battery, even recharge the starting battery if it is depleted.

You can also take a look at the Lithium Battery Isolation Manager: https://www.precisioncircuitsinc.com/product/lithium-battery-isolation-manager/

It will parallel the batteries for charging... But I don't think it will give you the parallel discharge you're looking for. Maybe use that and run the loads of the LFP battery?

 

[packrat1969]

Thanks for taking the time to reply. The Sterling unit appears to handle the charging portion, however it is way to expensive and didn't seem to handle the parallelling. The other unit PARALLELS both batteries while charging only.did I miss something?

 

[meetyg]

I don't quite understand why you want to parallel them?

Anyways, this DC-DC charger has reverse charging feature, meaning that if your starter battery goes flat for some reason, it can recharge it from the leisure battery:

12V 25A DC to DC Battery Charger MPPT System Kit Isolator Dual Battery

Gear up for your adventures with the ultimate dual-battery solution: the ATEM POWER 25A DC to DC charger. With its advanced features and superior performance, this DC-DC charger is the perfect companion to keep your batteries fully charged and ready for action, ensuring you never run out of...

atempower.com

BTW, this unit also does MPPT solar charging.

A friend of mine is planning on getting one these for his overlander.
So I can't tell how well this specific unit works yet, but the features are impressive. They also make some stronger ones (like 40A) if I recall correctly.

 

[packrat1969]

I have a work vehicle that will run its accessories until a battery-saver circuit decides the starting battery must be preserved, and shuts everything off. I just wanted to parallel separate battery to hold the voltage up for a longer accessory runtime. I'd like to have the LiFePO4 battery replenished while the engine is running but not subjected to any engine starting draw.

 

[mikefitz]

charger has reverse charging feature

All the reverse charging features only work if the house battery is being charged by something, solar or an AC battery charger. The house battery needs to have a higher than resting voltage to trigger the reverse charge.

FLA starting battery's accessory runtime power

A DC to DC charger will perform the charging of the lithium battery, the DC to DC is necessary to limit the current taken from the alternator.
To connect the batteries in parallel can be done safely with additional circuits and power relays, or something as simple as a manually operated switch, ( much less safe).

What power do the acessaries take and does the ignition have to be on for the acessaries to function?
If possible the ideal situation is to power the acessaries directly from the lithium battery and avoid the switching.

 

[chicagoandy]

Thanks for taking the time to reply. The Sterling unit appears to handle the charging portion, however it is way to expensive and didn't seem to handle the parallelling. The other unit PARALLELS both batteries while charging only.did I miss something?

Like I said, I'm not sure there's a device that does exactly what you're asking for. Paralleling Lead-acid and LFP together can be problematic as they both have different charging profiles, the Lead-Acid batteries have very low "float" voltages, so if they're left in parallel, the Lithium battery will get pulled down to that same voltage and will have very limited capacity. This is why people don't generally directly parallel these batteries. I don't know of any way that you can parallel them during discharge without reducing capacity of the Lithium battery.

But the first device I mentioned, the Sterling, will I think solve the problem for you. Yes, it's expensive, but there are cheaper units that run at lower amps. They may also work for you, but they may or may not provide enough amps to solve your problem.

For example here's an 8 amp Sterling that runs 200 British pounds, , they'll happily ship globally. But it will only send 4 amps back to your starter-battery. https://sterling-power.com/products...Ew5XIngIN08f6Y3nbQDL9k7vxDf248Zxuf7ulD2ECcjT0

You've got some options:

The Sterling unit is a Battery to Battery charger, and these are fairly common - but bi-directional ones are not fairly common, which is one of the reasons the Sterling is more expensive.

The second item I mentioned will parallel them only during charging, and separate them during discharge to avoid discharging the Lithium cell. You could do this, and connect your loads to the Lithium battery instead of the starter battery.