Can I parallel different ah lifepo4

[pyrofx]

Can I parrallel different ah calb cells. I have 170ah cells . Right now I'm in an 8s setup. It's actually in an off grid workshop. The cells are holdin 175 ah , and I have the opportunity to pick up some more , but they are holding around 100ah. They are a good price and may want to add to my bank later on.
I could connect the new bank to the power buss via another bms if need be.
Greg

[john61ct]

I'd treat them as separate banks, just combine while charge source is active, have A/B load switches. Maybe keep the healthier bank as Starter-Reserve, and put the House cycles on the weaker one.

[S/V Illusion]

If you combine them while charging, it's likely to result in overcharging the larger capacity batteries.

[pyrofx]

I figured it would over charge the smaller bank. But if I run a separate bms on each bank , it should cut off the charge when full.

[john61ct]

Yes, I disconnect from the charge buss at 3.45Vpc, no Absorb; as I said treat each as an independent bank.

[Cpt Pat]

I've had my 125 AH LiFEPO4 battery paralleled with a 165 AH AGM bank for 2 years and about 75 discharge/charge cycles. No problems.


LiFePO4 batteries should have all charge disconnected when (or before) reaching full charge, and the battery disconnected when (or before) it reaches its discharge floor.


If you have the lithium set up as a separate bank, how will the loads behave when the lithium is switched off at the discharge floor? You could use steering diodes to combine your lead-acid and lithium, but I've chosen instead to run the lithium in parallel. Here are my parameters: (LFP = LiFePO4 battery. LA = lead-acid batteries). Voltages are measured at the LA bank.


Charge:

13.8 volts, LFP switched off the bank, LFP charging ceases
14.4 volts, LA charging ceases.
13.4 (temperature compensated) LA float voltage.

Discharge:

13.2 volts, LFP is reconnected to bank in parallel.
12.7 volts, LFP is disconnected from bank.

The LFP is disconnected from the bank with a breaker switch at the end of each sail, and reconnected for charge immediately prior to next sail. There is no danger to leaving the LFP battery partially charged (they don't sulfate) but the calendar life is reduced by storing it in a fully charged state.

That's it. Nothing untoward happens when the LFP is connected in parallel with the LA batts at 13.2 volts. The LA batteries draw practically no current from the LFP at 13.2 volts. I've measured it - there is a "surge" of 2 amps that diminishes to zero in one minute (0.03 amp hour - if you prefer). For the first 100 AH (I don't fully charge/discharge the LFP), it does all the work. The LA batts just float. The LA batteries take over at 12.7 volts. Power is never interrupted to the bus.


I'm using a relay (https://www.digikey.com/products/en?...N2C-DC12&v=236) to connect/disconnect the LFP to the bank. I'm using a APO3 (http://www.aprsworld.com/apo3/) to disconnect the LFP at 12.7 volts, and a Victron battery monitor (https://www.victronenergy.com/batter.../bmv-712-smart) to both monitor the battery and de-energize the relay at 13.8 volts. The maximum charge/discharge current used in my application is 30 amps (I don't try to start an engine with the bank). The LFP is fused at 50 amps, and the breaker trips at 30 amps. If your current needs are higher, you'll have to use a different relay.


I suggest using the DPDT relay linked above, and wiring the two sets of normally open contacts in parallel. The DPDT relay doesn't cost much more that a SPST/SPDT relay, but it'll last longer with a set of connects doubled.

[john61ct]

Wow, great stuff.

Originally Posted by Cpt Pat

I'm using a relay (https://www.digikey.com/products/en?...N2C-DC12&v=236) to connect/disconnect the LFP to the bank.

That link does not work for me (Chrome on Android)

Maybe better to just use mfg model#s?

Is this it?
https://www.digikey.com/product-deta...Z269-ND/355287

30A seems very small to me.

[john61ct]

Here's a Mouser search 100-200A

https://www.mouser.com/Mobile/Electr...yzxg34Z1yo23r7

Note the $4 one!

Any applicable?

[john61ct]

Cross fertilizing

http://www.cruisersforum.com/forums/...id-206572.html

[Cpt Pat]

Originally Posted by john61ct

Wow, great stuff.

That link does not work for me (Chrome on Android)

Maybe better to just use mfg model#s?

Is this it?
https://www.digikey.com/product-deta...Z269-ND/355287

30A seems very small to me.

(This server truncates displayed URLs that are long. You need to click the link - not copy and paste it.)

Digikey part number Z269-ND. Google should bring it up. I prefer open frame relays so I can visually confirm operation. BUT DON'T USE THAT RELAY IN A POTENTIALLY EXPLOSIVE ATMOSPHRE (propane or gas fumes)! In that case, you'll need a hermetically sealed relay approved for explosive atmospheres. I prefer metal cases, plastic can crack.

If you're drawing more than 30 amps from a 100 A/H battery (0.3C), you'll need a much more expensive relay, like Blue Sea Systems High Amperage Solenoid model ML-RBS. If you're continuously drawing 100 amps, you shouldn't be using a 100 A/H LFE. That's 1.0C, and the life expectancy won't be much better than a LA bat.


I'm sure you already know this but, don't charge the LFP when its temp is below freezing.

[rgleason]

Cpt Pat,
I am sorry, I have not followed your boat's configuration.
Do you have a BMS?
What LFP?

Are you using SolarPV primarily and not an Alternator? The reason I ask is the 12v 30a relay.
What size is your Alternator?

Do you wire the APO3 (LFP drop below 13.2v) and BMV-712 (LFP rise above 13.8v) in series so if either one breaks the relay disconnect the LFO?

Do you know if the BMV-712 can also be used for the low limit? -apparently not, it is a setting but it only has one set of contacts?

Why doesn't Victron expand the BMV-712 to provide two sets of contacts for the LFP, one for High and one for Low?

I assume your Solar MPPT and Alternator Regulator are protecting the LA battery.

What are the potential problems with this approach as compared to having separate discharge and charge buses?

Thanks. Rick

[pyrofx]

Great info guys. Thank you.
I'll keep all this filed . When the boat day comes it's the best discussion yet.
Right now I'm land based. My setup is for a workshop that is 12 miles from power . I'm using solar that charges a lifepo4 bank, no lead.
The concern is charging the parrallel strings. One bank ie 170ah calb cells , and the other is 100ah.
Greg

[thomm225]

Originally Posted by S/V Illusion

If you combine them while charging, it's likely to result in overcharging the larger capacity batteries.

I'm thinking it's the other way around.

The battery with the lower capacity will charge up first then the charger will shut down before the larger capacity battery is completely charged which over time could damage it

[john61ct]

When batts or banks of the same chemistry are parallel combined, they may have slightly different voltages internally, in which case current will wastefully flow from one bank to the other.

But on the wire, there is one circuit thus only one voltage for the charger to know about.

In a "hit voltage then stop" CC-only charging scenario, I guess that means both banks (packs) will be at nearly the same SoC.

Only if Absorb/CV is held for a while would any different-charging damage be inflicted.

The hybrid LA+LFP issue is off-topic here, so discussion of that should take place in the thread I linked to above, or a new one if you like.

Personally I would still prefer treating the two packs as separate banks once charging is complete, yes at the cost of a little extra complexity.

[thomm225]

To simplify, the lower capacity battery will charge up first and the charger will see this and shutdown.

The larger capacity battery(s) gets left behind and doesn't get a full charge.