Mixed bag of batts to one Inverter?

[Swamplizard]

Working on yet another budget LifePO4 project. Goal is to have more 12V capacity for a small room air conditioner and use the battery packs I have.

I have Three (3) 12V battery packs each using the exact same BMS (each). One Pack is 175AH, one is 75AH, and one is 50AH. Batteries are different manufacturers for each pack but consistent within a pack.

Inverter is a 1500 and drives the A/C easily as it is low seer, small, low power consumption, new, and can run off any of the three batteries one at a time.

Any reason why I can't combine the three batteries by connecting them to a central power bus? Fuse to Inverter of course!

Can One Charger connected to the Bus and charge all three batteries (I assume BMS will stop the charging when each battery is full and disconnect them when they are spent).

As always, Thanks!

 

[sunshine_eggo]

Any reason why I can't combine the three batteries by connecting them to a central power bus? Fuse to Inverter of course!

Yes. Each battery is rated for a different current. You would need to ensure that all batteries are operating within specification.

Can One Charger connected to the Bus and charge all three batteries

Yes, but with the same criterion - each battery must charge within its specification.

(I assume BMS will stop the charging when each battery is full and disconnect them when they are spent).

This is a great way to design for disappointment. You want your devices to operate inside of the operating limits of the BMS - not rely on it for routine cut-off.

 

[Swamplizard]

Thanks - Interesting inputs.
Perhaps I should have an ABC switch and simply flip between batts as needed.

 

[Swamplizard]

This is a great way to design for disappointment.

Can you clarify this? Do you mean it will burn down the garage or that it will not allow me to use 100% of each battery bank (which I can live with as long as it is safe? Thanks!

 

[sunshine_eggo]

Planning a system that relies on routine disconnects for charging and discharging runs the risk of damaging the BMS(es).

A system should be design to operate INSIDE the operating parameters of the BMS... say 12.0V to 13.8-14.2V or so.

Risk of fire with LFP is very low.

 

[JRH]

Planning a system that relies on routine disconnects for charging and discharging runs the risk of damaging the BMS(es).

A system should be design to operate INSIDE the operating parameters of the BMS... say 12.0V to 13.8-14.2V or so.

Risk of fire with LFP is very low.

Would the first sentence of that last reply include using an IMO switch…to disconnect the pv array…
I do that before thunderstorms…
or were you talking about just battery’s and their connections..
I wasn’t clear how you meant that..
thanks, jim.

 

[sunshine_eggo]

Would the first sentence of that last reply include using an IMO switch…to disconnect the pv array…
I do that before thunderstorms…
or were you talking about just battery’s and their connections..
I wasn’t clear how you meant that..
thanks, jim.

Edited for clarity... "BMS disconnects" i.e., don't rely on the BMS to cut charges and discharges for routine operations.

 

[Brett V]

The BMS is an autonomous safety device operating in the background. It’s not a routine control device being operated in the foreground.

 

[sunshine_eggo]

The BMS is an autonomous safety device operating in the background. It’s not a routine control device being operated in the foreground.

That ^!

 

[JRH]

Edited for clarity... "BMS disconnects" i.e., don't rely on the BMS to cut charges and discharges for routine operations.

Gotcha.

 

[Swamplizard]

Thanks all - always helpful!

Perhaps I am better off taking the "scrub" mismatched batteries and make one larger lower AH 12V and call it a day? ie. combine the 75AH and 50AH cells into a bigger pack and have an ABC switch - A to Load, B to 175AH pack, and C to 50AH "mutt" pack.

 

[Swamplizard]

any feedback?

 

[sunshine_eggo]

Thanks all - always helpful!

Perhaps I am better off taking the "scrub" mismatched batteries and make one larger lower AH 12V and call it a day? ie. combine the 75AH and 50AH cells into a bigger pack and have an ABC switch - A to Load, B to 175AH pack, and C to 50AH "mutt" pack.

Many ways to do it. As long as you understand the implications with each option, there's not necessarily a best option when trying to integrate mismatched components.

Properly paired cells can minimize issues, but a significant disparity in C rates may cause cells to degrade faster.

basically, whatever configuration you decide, the following two criteria are key to success/longevity:

1) Are all elements operating within their "C" rate specification?
2) Are those C rates similar to each other?

#1 is mandatory. #2 ensures maximum longevity.

Let's assume 0.1C:

175Ah = 17.5A
75Ah = 7.5A
50Ah = 5A

So, if a 30A load on the battery bank sees a proportional distribution of current, you're all set.

A switch mechanism is workable as well. However, that presents additional issues. Switching from one to the other with an inverter as a load may trigger BMS over current protection unless the inverter is powered off first.

 

[Swamplizard]

Good stuff Eggo - Thanks. The use case is supplemental A/C on a boat. Every "blue moon" we will want A/C for sleeping or to cool down cabin. A/C is new and very efficient so 1500 Watt inverter drives it without issue and very low over-all Amps even when compressor kicking in. Shutting down inverter before I switch batteries is a non issue for us. BTW this lets me get rid of 400lbs of 6V lead deep cycle house batteries....NICE!

 

[Swamplizard]

Working like a charm after I upgraded two things - one the inverter was 20 years old and when I touched the case it zapped me a bit so swapped for new 2400. Two, I had the wrong Amperage fuse/breaker. 40AMP worked fine for A/C, Fans, and small items but if I tried coffee pot or microwave (even on low power) breaker would pop.

Got some good advice on here and upped it - now everything works though, of course, I only run one power sucking device at a time.

As always, thanks for the help and for not being condescending to a relative newbie. Great Site!

 

[Swamplizard]

got my hands on some new hardware last night - picked up a Daly blue tooth 4s 12v 200A which will be better suited to this boat project. Plan now is to:

• use 2500 watt inverter
• upgrade power and ground bus cables to handle 200+ Amps
• use 200 Amp breaker
• use Daly 200 Amp BMS with ABC switch
  • A to 175 AH battery
  • B to 75 AH battery (probably upgrade it later down the road)
  • C to inverter

Inverter has a remote panel which shows battery level and shuts down/turns on inverter. Will shut down inverter before switching batteries.

Not perfect but should be an over-all better solution. When I see some decent 280-300 cells around I will build a new bigger battery as the primary.

 

[JoeHam]

I had the wrong Amperage fuse/breaker. 40AMP worked fine for A/C, Fans, and small items but if I tried coffee pot or microwave (even on low power) breaker would pop.

As always, thanks for the help and for not being condescending to a relative newbie. Great Site!

You may already know what I say below as I point out that the primary purpose of a breaker is to protect the wire.

Normally the proper size breaker is installed on day one and it’s not a great idea to put in a higher Amperage breaker without increasing wire size appropriately. You don’t want to just put in a larger breaker to run a heavier load.

Perhaps you had oversized wire with an undersized breaker from the start ?

 

[Swamplizard]

yes oversized to start. Thanks for double check