What B2B lithium gizmo am I looking for?

[hour]

Hopefully my diagram explains what I can't manage to do without producing a wall of text (it still happened).

I was going to be paralleling a low state of charge 18ah aux battery to a 200ah(2x6v) golf cart battery floating at full charge, both lead acid, and that apparently was okay since the resistance of the 18ah battery would prevent some enormous current transfer. With lithium, I do not know.

What I do know is that my 90ah lifepo4 house battery is going to be kept at like 13.3-13.35v and be full most of the day, and I'd like to connect a potentially near-dead 32ah lifepo4 battery to it. Alternately, I might charge the 32ah inside to a higher voltage than the house battery, set up camp, and put in parallel.

I'm aware that in this approach the 32ah battery will never reach a higher voltage than the house battery, as regulated by its solar charge controller, and that's okay. If I can bring a dead 32ah battery up to 13.35v I'll be happy. I just don't want to have 10 gauge wire maxing itself out when a flat battery is connected to a full one, nor do I really want to have a 13.5v 32ah battery trying to equalize with a 13.35v 90ah battery.. though that outcome wouldn't bother me as long as its safe. The 32ah does have a 100a BMS, but even if it's grossly overrated by chinese mfg specs I still don't want anywhere near 50a charge going in to it.

Would a DROK step up/boost 5-amp converter work for this? Input voltage 13.35, output 13.36 (or anything slightly higher, whatever it lets me), and be current limited to 5 amps? Or will the dead 32ah try to suck a ton the second they're paralleled and blow the DROK?

Thanks for ideas, part suggestions, anything!

 

[jonyjoe101]

I've connected a 27ah li-ion 3s16p (11.1 volt) to my 220ah lifepo4 13.3 volts (14 volts while charging from solar). The 27ah was around 11.8 volts when I connected it directly to the 220ah lifepo4 battery, I did have a dc wattmeter between both batteries to measure current being transfered. The most amps I saw was 9 amps going to the li-ion when the lifepo4 was being charged by solar, if a cloud pass by, the amp transfer was around 5 amps. The wire I use was about 12 gauge and it wasn't getting hot or even warm.

Thats the method I use to charge both my 27ah and 65ah li-ion, they both have bms and overvoltage relays, I just plug in to the lifepo4 house battery. As long as the lithium you trying to charge has a bms, it should be good. Lifepo4 to lifepo4 the voltage are so close together, you shouldn't see any massive amps transfer.

If you have a dc wattmeter, you can monitor in realtime and if you see too much current passing you can disconnect right away. But I never seen it, I even connected my 28ah agm lead acid to the lifepo4 to charge it and don't see more then 6 amps being transfered.

The only time I use a DC charger is when I want to fully charge the li-ion, the DC charger is a 2 amp charger. At 5 amps the bms cutsoff charging to quick, but at 2 amps I can get the battery almost all the way to full. Slow (low amp) charging will also work with the lifepo4.

 

[hour]

Lifepo4 to lifepo4 the voltage are so close together, you shouldn't see any massive amps transfer.

I'm assuming you only run the QNBBM on your primary larger pack? Does your BMS on the 27ah only balance when cells are up to 3.6v or whatever most of them claim?

Also re: the quote, is that sentence correct? I'm thinking 12.5v 32ah battery meets 13.35v 90ah battery... is that truly close? There's a lot of energy between those two voltages for either battery.

Are you aware of any consequence if I forget to disconnect them in to the night... They'd be running the fridge together til the sun came up, fridge being powered directly from the 90ah battery and its affiliated protection stuff, and the 32ah battery probably supplying power to the DIY boom box it resides in. I know it'd affect my BMV's calculations/calibrations which are set for the 90ah battery, but if it's safe then hell yeah. 60w power draw from fridge, and barely running at night even in the middle of summer at high elevation. Scenario would be... not wanting or needing to move the boom box closer to the fire at night but still wanting some background music

 

[john61ct]

Better to isolate out the bits involved with one specific question.

So just looking at "connecting in parallel two batts"

Yes, if lead is involved on either side, one full one dead is NBD, but robust wiring is of course important.

I would test measure the max current transfer with very fat cables in worst case conditions and time-log the transfer to get your production-use AWG and fuse capacity.

LFP to LFP is a whole nother thang, I would use a DCDC charger like Sterling BB series, 40-60A, unless you get a specialist to figure another way to limit current, but I doubt that latter would save money.

Inexpensive gadgets if they have any current limiting protection, do so by disconnecting, act like a fuse, likely need a manual reset.

Even lots of expensive gear just lowers voltage, it is very rare for charging V to be maintained accurately and just the current safely throttled, certainly $200-300 would be cheap so might as well get the Sterling.

Or as you say get them both to the same voltage separately, before joining.

Note the best for efficiency is just join with charge source active, that raising circuit voltage above the higher batt keeps the energy where it's stored.

Flowing energy from batt to batt without a charging source active, that is crazy inefficient.

 

[john61ct]

Close in voltage is within .1V, LFP to LFP that can still push a fair bit of current.

Note your contactor points will wear, need proactive replacement or can weld closed.

 

[luthj]

You could just find an adjustable buck/boost combo voltage converter. Set it to output a constant voltage of say 13.5-14V. Obviously pick on that limits its output current. Something in the 5-10A range maybe? That would allow you to move as much power around as you like, regardless of the voltage levels on either side. Of course you loose some power to heat in the converter.

 

[john61ct]

And yes tiny Ah sizes will keep rates low, spikes not held for long.

But from a big Full LFP to even a small empty LFP, rolling the dice there. . .

 

[john61ct]

Obviously pick on that limits its output current.

See above

 

[luthj]

To Limit the current, it would be fairly easy just to place modest size wire and/or fuse to act as a resister. With a bit of math you could easily limit your current to say 10 A with a fully charged and fully discharged lithium on each end .

 

[john61ct]

I'd want to be darn sure of that math, risky business.

Actually measuring, start with less difference, work your way up to more, watch out for heat.

High enough amp rates you're also reducing battery lifespan. . .

 

[hour]

Damn, what a rollercoaster of emotion this has been. Joy that jonyjoe has done it with lithium to lithium and a bunch of other tests, sadness at John's response, hope that a $20 solution exists from luthj's post, sorrow seeing it not be endorsed by John and then (?) dismissed by luthj with the mention of wire gauge games.

Honestly, buying a B2B charger (sterling, redarc, renogy) for this purpose seems like it HAS to be overkill. The voltage regulation of a B2B won't benefit me at all, The current regulation of a B2B, say a 20 amp unit, is still more than I want or need to pump in to the 32ah battery.

To Limit the current, it would be fairly easy just to place modest size wire and/or fuse to act as a resister. With a bit of math you could easily limit your current to say 10 A with a fully charged and fully discharged lithium on each end .

So comparing your first response to this one, I'm guessing the DROK type units don't restrict current then? I'd much rather spend the $15-20 for a step up converter then play the wiring gauge game... sounds like it could take days to figure out for all the various states of charge between one battery and the other - and their current draw with various wire gauges. But I'd rather do that then spend hundreds on a B2B that has such a limited use case.

Take for instance my hobby charger, ISDT Q6 Pro. It accepts an input voltage of 7-40v and outputs 0-30v. It costs $50.
Efficient or not, you can charge one battery from another using this unit. If you supply it with a 13.35v 90 OR 32ah battery, you can plug in a completely dead 12v lifepo4 of any size and nothing dramatic happens, it just charges at whatever you have the max charge current set to.

The fact that this unit costs $50 and can do all of that, makes me think that inside somewhere is a $5 part. The Q6 can boost/buck which if anything I'd only need the boost portion of, but in reality probably neither. This is actually pretty standard of most hobby chargers I've found. It's not proprietary technology or anything..

 

[john61ct]

There are people I'd trust to do the limiting-current-through-too-thin-gauge wire.

But it would only apply to the X max amps that particular battery pair can produce, over Y minutes.

Apparently Z wire length only relates to Voltage drop, not thermal issues.

Bit too redneck engineering risky for me though, but not saying it can't be done.