Some LifePO4 BMS and charging questions mixing 12 and 24v

[willo]

So I'm adding some high performance bits to my truck and have been looking at various charging options.
I've got some... off the wall ideas that I'm sort of considering. I know the mad scientists here may have some useful ideas!

1) Main charging source is the 12-14v car battery system.
2) I've built a pair of 12v Headway LifePO4 packs using the 8Ah HP cells (200A discharge each in 4p4s packs, so 800A max. :O )
Figure on 1.3Kwh to work with at 80 percent DoD.
3) I have a couple basic BMS that I picked up when planning on pure 12v, no big deal they were cheap.

Problems to solve:
1) My winch will be 24v. This thing can pull say 250A at high load (worst case). (7hp 24 RED Winches OX Motor) - That load won't go through the BMS, but I will add LV cutoff to the control circuit. A given winch pull would likely be 1-4 minutes if that.
2) Also adding in a 24v air compressor - it was like half the price of the 12v and it got me on the whole 24v idea.
3) I'm gonna need some significant charging for winch sessions. I can always wait 5-10 mins to recharge. That said I don't usually need to winch a ton, but once in a while it's like 4 pulls in a row in the snow.

Ideas I'm considering:
1) 12 to 24v DC-DC charging. This is the easiest, but provides the least charging output. The simplest example of this would be a Victron 12/24 DC-DC charger, takes in 12v, outputs 24v (tunable for lifepo4) maxes out at 15A. this thing is around $270ish with bluetooth. Most loads would go through an 8s BMS with additional temperature cutoffs from a custom battery monitor.

2) A pair of Isolated DC-DC chargers that are built for 12v. This feels a little weird but would maximize efficiency and optimize available charging. For example, a pair of the 12/12-30A from above. edit: there are many cheaper 12/12 options. If I were buying a pair of the orion-tr's I'd just buy a pair of the 24v. Use each to drive one 12V pack. In theory, since these are isolated chargers, I should be able to stack them. This doubles my charge rate to a total of 30A at 24v.
I figure I'd want a 4s BMS on each pack and the chargers would each feed into a BMS.

I may combine this with an 8s BMS to drive 24v loads through. This could also be used to over-ride the winch load, which would be directly connected (aside from solenoid switching for on/off). Again, I'd have a custom battery monitor to monitor battery temp and provide extra cutoffs.

3) All options will end up with a solar controller, charging the pack at 24v. It's one more source, but probably a factor for my winch discharge/recharge events.

In the end, I may just be overthinking this. For example, I can parallel the 12/24 chargers and easily double the charging and have a simple 24v system to work with.

 

[willo]

I'm going to say that I'm probably just going the 24V route to be done with it. Once I have a feel for my actual winch discharges I'll know if I need more charging. That said, I am curious about the idea of using a pair of isolated 12v chargers. It's worth kicking around at least.

Cheers.

 

[john61ct]

Can the vehicle be converted to 24V, use a DCDC converter 24-->12V if necessary for a 12V circuit?

Or **adding** a separate 24 alternator?

Definitely want that alt cranking out big amps to support the winching sessions, not just fed by battery storage.

I don't think you did the math right for your little Headway bank capacity.

 

[willo]

I did the math corretcly, but half awake posting I put up the wrong pack info.
I've got 64 headways in a 8s8p configuration. (not 4p, which my brain said since the packs are physically 4 wide)
8Ah per cell * 8 = 64Ah
64Ah * 24v = 1,536Whr.
80%DoD puts that around 1.3ish Kwh. (This part I had right)

Each Cell is rated at 200 Amp discharge * 8 = 1600 Amp discharge max for the whole pack.

 

[willo]

As for the other questions, I do not want to combine the systems. I actually want a pair of separate electrical systems. Since I'll be separating the charging systems anyway, there's no compelling reason to convert the car away from 12v.

In the end if I decide I want a 12v winch, then the 24v system can just go onto my F250 diesel.

 

[john61ct]

So can you add a separate 24 alternator?

> Definitely want that alt cranking out big amps to support the winching sessions, not just fed by battery storage.

Put an ammeter on your winch under load and you'll see.

Running such a tiny bank at a very high C-rate unsupported will not only drain it very quickly, but greatly reduce its lifespan, you'll see voltage fall off the cliff quickly.

Headways aren't A123, nor CALB, Winston/Sinopoly or GBS.

Better than K2 maybe, Full River, for sure, probably VariCore/Liitokala.

Otherwise maybe use a little inverter genset + high-amp shore charger, or not expensive to rig a DC-direct gennie.

https://www.youtube.com/channel/UCpNKkVULsIKilHs3qqp_t7w

 

[JackFlorey]

3) I'm gonna need some significant charging for winch sessions. I can always wait 5-10 mins to recharge. That said I don't usually need to winch a ton, but once in a while it's like 4 pulls in a row in the snow.

Use a DPDT relay to connect each 12V battery to your main system alternatively, so first you are charging the bottom battery then you are charging the top battery. If you do this do NOT ground the bottom of the 24V battery to the chassis. It's also inherently bidirectional so you can use the 24V battery to charge a dead 12V battery.

 

[john61ct]

Have a look at Yandina's "trollbridge" autoswitching12V / 24v

from the engineer invented the whole VSR / ACR / combiner concept . . .

tech@yandina.com

 

[JackFlorey]

Have a look at Yandina's "trollbridge" autoswitching12V / 24v

that also works, although you cannot use them both at the same time. (i.e. charge @ 12V use @24V.)

 

[john61ct]

Well technically the "bottom half" 12V **can** continue to accept charge while the full 24V bank is discharging.

But yes, it's designed for rigs where most of the time paralleled 12V is what you want, and the higher power needing loads requiring the 24V ooomph are only occasional and relatively short-lived.

Personally with a HD winch operating, requiring concurrent support from an active ICE energy source, I don't think the TrollBridge is the solution for OP.

But it's worth opening a discussion with Ann-Marie, she's usually very responsive and might have an OOTB solution.

 

[willo]

Yeah, this motivated me to run some load numbers up.

Let's say that a given pull with this motor will be all of a minute or two.
Let's call it 200A at full load. I don't have good numbers yet. I suspect my peak may be a hair higher, but average will end up being lower - Most often I'm winching to pull someone out of a ditch or snow bank - those aren't usually hard pulls.
wh = Amps * Minutes/60 * Voltage

Load Amps Time Minutes Voltage wh
200 1 24 80
200 2 24 320
200 3 24 720
200 4 24 1280
200 5 24 2000

My math may be horribly wrong, but I think I've got it. It looks like at a load of 200A, I'd burn my pack down in a bit over 4 minutes. With my old winch setup, winch pulls can and do take quite a while. With the new winch, it's waaaay faster.

Recharging the pack at 15A, in a perfect world, will take something like 15 minutes if I drain it 80% DoD. (yeah it'll be a bit slower than that)
Charge Amps Time Minutes Voltage wh
15 5 24 150
15 10 24 600
15 15 24 1350

So eh, not that bad.
I'm going to agree with the judgement that my pack is a bit small. I'll add some shunts to monitor actual loads and see where I'm at. I'm also going to test running it at 12v for a low power mode. In the case of longer sessions, I could switch it over to use the standard battery system....

 

[john61ct]

At 64Ah, pulling 200A continuous will be stressful, not optimum for longevity, and you will as I said see voltage drop off very quickly without support from ICE input.

Get a coulometer with a 500A shunt to actually measure the load, Ah consumed over several 5min. Use Ah to measure, only Wh if comparing 12V to 24V.

A 15A charge rate will be good for longevity, at least as long as the cells are kept warm externally when ambients go below say 20°

Don't rely on internal resistance from usage to "warm them up from inside"

If you discharge 50Ah, it will take well over 3 hours to replace it.

I could switch it over to use the standard battery system....

In which case you will want a very robust bank to support the high amp deep cycling there too

 

[willo]

Yeah so of course I woke up this morning pondering a second alternator. The issue I have with that is that I'll want to run that into a dedicated 24v AGM bank.
This isn't an issue really, I can redirect my headway pack to my camper project.

 

[john61ct]

Just put the LFP in parallel with an ACR, then an HVC set at your desired termination point, say 27.6V

LFP gets isolated, AGM keeps going.