Adding 310ah to 280ah system

Tomtennant

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May 21, 2021
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7
This is a home system being used for emergency power only. This is not used daily (or even monthly). I started this project after the Texas freeze and was without all power for a week.

I purchased a 24v inverter and have 8 x 280ah LiFePO cells in series. I have a Daly 8s 250a BMS and a separate 30a 24V (29v) charger that i power from the grid. There are no solar panels involved in my setup.

I wanted to have a little more backup so I purchased 8 more cells. However, I am getting 310ah cells this time (it is what it is). I've read you shouldn't combine batteries if they have different ah ratings as the smaller one would drain faster and the larger one wastes energy trying to change the smaller one. Eventually cells can become damaged due to the difference. Most discussions talk about combining vastly different sizes - like a 50ah battery and a 200ah one.

Should I just create a separate 8s 24v battery and keep them completely separate or can I make an 8s2p with these cells? Are these cells "close enough" that for my purpose it will be "good enough"?

If I do the separate 8s battery, I'll have to get a different BMS and figure out how to connect them to the inverter and charger so the 280 set doesn't pull the 310 set down.

If i combine them into an 8s2p setup, will the single charger and single 8s BMS charge each cell correctly?

What else am I missing?
 

sunshine_eggo

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Oct 26, 2021
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Wire them in parallel in accordance with the best practices in the Victron Wiring Unlimited handbook in the Resources section.

Once deployed and in operation, use a clamp ammeter to measure the current through each battery during a discharge or charge. As long as the current they are experiencing is within the cell and BMS limits, you'll have no problem. You'll almost certainly see the 310Ah share more of the current than the 280 if you make good connections.
 

rickst29

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Mar 27, 2021
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Reno, NV
In a smaller configuration, I have 4 x 105ah cells as one battery, running in parallel with 4 x 120ah cells as another. My differential in battery capacity is about 14%, while yours will only be about 10%. That's very close, and I would be surprised if you experienced any significant "out-of-balance" situations in either charging or discharging.

In discharging from fully charged battery packs, the higher voltage wins - so they are drawn down together. Later, when operating in the 12.8V "nearly constant" portion of the State-of-Charge curve, they also work fine (with slightly more power drawn from the 120ah unit during discharge). At the end of charging, BMS clamps the smaller battery into 'balance mode' just a very short time before the larger battery also reaches balancing voltage.

The total resistance within my two battery packs is dominated by the resistance of the BMS units, rather than the cells, and it's roughly equal on the two packs. I will recommend staying with the same brand and design of BMS units, in order to make the resistance of the two packs nearly the same.
 

Tomtennant

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May 21, 2021
Messages
7
Great, thanks! I will create a new 8s battery with these cells and get another bms then write them in parallel. I hope this system will never be used but will just be there if needed.

I've read the Wiring Unlimited book by didn't see much about batteries with different ah ratings. Lots of good info there!
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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Oct 29, 2019
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Rural NE Ontario Canada
I run with 175AH & 280AH and with that 105AH difference it gets "quirky" to put nicely. Generally up to 50AH difference between packs in a bank won't present many issues but beyond 50AH things have to be planned for.

Typical situation example. All packs charging to "designated" 100% (absorb to 28.2 and floated to 27.9).
As the 5 packs charge, they will take different Amps relative to SOC & AH Capacity. The 280AH cells take more than the 175's.
As the 175's reach "full" their resistance increases and Amps taken drops, at this stage, the additional Amps go to the 280's which happily take it and because the cells are already up they top off quickly. END-AMPS/TAIL-CURRENT is set at 0.05C and uses the Largest Pack in the Bank (Not cumulative) which for 280AH is 14A.
--> The GOTCHA ! Even though the 175's reach full @ 28.2 and the amps drop, they are still taking in charge. The cells being at 3.525Vpc is HIGH and they do deviate up to 200mv (NB: The 175's are Used (abused) EV Grade LFP). ANY charging above 3.450 will certainly cause cells to deviate with Used, Bulk or B Grade this is not unexpected. The NASTY, if the BMS is not programmed to handle such cases it will trip & cutoff or to start cycling on/off and many charge systems will not like that !

Discharging issues.
There is no issues when discharging hi or low C-Rate while all packs are within the 3.000-3.400 working voltage curve. You will see that the smaller packs will output less Amps than the larger packs... That is perfectly normal as it is relative. Once you start to reach low SOC (around 25%) the smaller packs will deplete before the larger ones obviously. The larger packs will in a sense level up the smaller packs but this is like a Surface Charge and wasteful.
The Gotcha on Discharge: If the batteries are too far apart, the larger ones will attempt to backcharge the smaller ones at low SOC, unfortunately, the trade off is a general loss of capacity that increases the lower the SOC goes. AGAIN, this is NOT SIGNIFICANT if the packs AH rating are close together.

In closing I also feel I should add one more item...
When assembling/building packs NEVER EVER Mix different cells ! Always use identical cells IE's EVE 280's or Lishen 272's do not intermix them within a pack. BMS' are MilliVolt and MilliOhm Sensitive and they measure each cell and calculate IR to do their jobs, different brands, sizes and grades do NOT play well together within a pack. One Brand, One Cell Type per Battery Pack.

Hope it helps, Good Luck.
 
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