DCDC Charger - Use LiFePO4 to Charge Starter Battery? Charge House Battery with engine off?

[John Holway]

Hi... From stupidity I've over discharged my lead-acid auxiliary batteries and they have broken down. Looking at getting a new system, and am very interested in these lightweight Lithium Iron Phosphate batteries. I think I'll only need 1 x 100AH battery. I have a t1n sprinter with a "dumb" alternator. I have 1 x 100W solar panel.

My current setup has a cable running between my house batteries (which are now dead) and my starter battery, and has a switch between them that I can toggle on and off only when I need to. I loved this setup because I could send voltage both directions, whether the engine was running or not. ( I also keep a jump pack in the van at all times). If I wanted to pump my subwoofer for a long time and not worry about draining my starter battery, instead of turning the engine on I'd just flip the switch on. If I drained the house battery in the night, or was worried that I might, I'd flip the switch on before bed and leave it on through the night (leaving engine off), and the starter battery would give the house battery just enough extra V to keep phones charging and heater going (perhaps that was bad practice).

Anyways, if I get a LifePO4 battery, I'll need to put a DC to DC charger between it and my starter battery, correct? If I do that, can I send charge back to the starter battery if I want? And, if I have the DC to DC charger, is there one that will send my LifePO4 battery charge even if the engine is off, for those rare midnight situations?

Thanks for the help!

John

 

[JohnSow]

Hi John,
I guess you have a van with 2 sets of batteries and one set is cooked from over charging. You also have a 100W (not 100A) solar panel?
If yo go for a LiFePo4 battery as you suggested then a BMS will have to be incorporated (either you buy 4 100A cells + a bms and just wire it in?) or you worry about doing this simple wiring and pay a huge premium to buy a all in 1 12V 100Ah battery with BMS and Blutooth etc.? Your choice.
However to charge between batteries is simple. You can use a $10 diode so that your "service battery" always charges the start battery or you can use a manual rotary switch from "west marine" or boating shop etc. that allows you to switch between B1, - B2, - Both, or OFF.
Or you can use small switches to control a couple 12v automotive relays that can connect between batteries manually with a small switch.

 

[RudyGreene]

Redarc has a kit that wires starter/house batteries this way. They use a second positive cable and a "smart" isolator. You could use their wiring guide as a template for your own DC charger: https://www.redarcelectronics.com/us/jump-start-dual-battery-kit

Sterling has started selling a bi-directional DC charger that might work for you, but it's not available in the US yet: https://sterling-power.com/products/12v-to-12v-120a-dc-to-dc-charger-bb12120

 

[JohnSow]

On various sites including aliexpress for example you can buy various cheap voltage regulators that you set the exact voltage (on charge and off charge voltage) as you like and that could be used to link your start battery until it is 50% or 12.0v before it cuts off using the starter battery to save the power to start in the morning. This way lack of memory not to flatten the start battery is less of a problem. You could also use another one to set a maximum voltage so you never cook another battery too? Cheers John

 

[JohnSow]

I agree with Rudy that Sterling chargers are pretty good and used a lot on yachting. They are sold in the US.
However if you have 115V~ or 230v~ to charge batteries then the most economic way is to buy a 5A to 50A SMPS and set the charging voltage you want. These devices are adjustable voltage and cost about $1usd per Amp you need and also known as LED drivers or 12V power supply. They all have 2 to 3 separate outputs that allow you to charge 2 to 3 separate batteries with the same charger. Hope this helps John Sow.

 

[JohnSow]

This is what you find if you type in SMPS (Switch Mode Power Supply) basicly how all modern battery chargers work today.

 

[John Holway]

Wow, so much info. Thanks so much guys. Many ways to solve a problem. And Yeah that bi-directional charger is sweet but yikes pricey. Also, it seems to only send power back to house battery if alternator is on. Would like for it to send power to lithium house battery when engine is off as well.

Hm, I have much to consider...

 

[fratermus]

if I get a LifePO4 battery, I'll need to put a DC to DC charger between it and my starter battery, correct?

Not necessarily. If

• you forsee the need for self-jumpstarting, and
• your setup is amenable to direct-charging LFP

... then a relay-based "isolator" of some kind may be preferable. I use a VSR that has a button which parallels the banks for a few mins even with engine off.

EDIT: the existing switch would work if you are careful to turn it off when not needed, and turn it off when the Li bank is fully charged.

If I do that, can I send charge back to the starter battery if I want?

DC-DC are one-way (starter -> aux) unless they have self-jumpstarting or starter battery maintenance features.

Of course one could wire a relay around the DC-DC and manually activate it when needed. Redarc describes this in their manuals IIRC.

And, if I have the DC to DC charger, is there one that will send my LifePO4 battery charge even if the engine is off, for those rare midnight situations?

IIRC the Ctek 250 series and the Renogy DCC series both have starter battery maintenance. Some Sterling manuals state they do, but AFAIK they do not (pre-2022 at least).

The Ctek+smartpass does auto-selfjumpstart when the starter battery voltage is low.

Check manuals for details rather than depending on some internet rando like me. Especially me.