diy solar

diy solar

Paralleling different capacities - at cell level or pack level?

meetyg

Solar Enthusiast
Joined
Jun 4, 2021
Messages
1,072
Well, it's a long story but I'll try to make it short:
After an order fiasco, I'm ending up with 8 x EVE LF90K (90ah) Lifepo4 cells, and another 8 x EVE LF105 (105Ah) cells.

I have a 24v all-in-one inverter on order and want to build a battery for it.

Although I am a JBD BMS fan, this time I decided to go JK 200A BMS because of the 2A active balancing.

Anyways, my dilemma is this:
1. To build 2 separate 24v battery packs, each with its own BMS, and use them in parallel
or
2. To parallel each 90Ah cell with a 105Ah cell, and build a 2p, 8s 24v battery (total capacity of 195Ah).

Yes I know that neither are ideal, but option 2 sounds weird. The reason for option 2 is of course cost and simplicity of the build:
1. Both cell types are same dimensions (or at least very close).
2. Don't need to buy another BMS (the JK is almost $100)
3. Less balance leads, only one battery to monitor, etc...

The main goal is to make good use of both cell types. Mainly for more capacity, I don't really need more power (not planning on maxing out the 3kw inverter).

Which path would be the best in terms of cell life / cycle life ?

Or should I just give up on the idea and find something else to do with the 90Ah cells (not much that I can think of, already got some 12v Lifepo4 packs of various capacities lying around).

You thoughts and opinions are appreciated!
 
Save yourself LOT of trouble.
Build 24V/90AH & 24V/105AH battery packs and Parallel them with matching cable lengths & a FUSE on each. 15AH Difference is not significant enough to cause any issues. Over 100AH Difference and things start to get screwy... I KNOW this as I have a pair of 174AH next to 280's in one parallel bank and that 106AH difference does toss in a few curves to work around. Any more and it would be more complicated and the end result would not be what one hopes for.

One BIG Rule, is never mixing different capacity cells into One Pack, the Lowest Common Denominator will rule in such a case, so effectively your 105's would end up being no more than the 90's
 
I run a 220 and 280aH 48V, two BMS's, two breakers, two class T fuses, paralleled together. Works good.
 
I run a 220 and 280aH 48V, two BMS's, two breakers, two class T fuses, paralleled together. Works good.
BTW, how did you parallel them? With a bus bar or just cables?

How did you size the breakers?
I already have a 160A DC , 2pole breaker on order.
After your suggestion, I wondering if I should go with 2 breakers or 3 (1 "main" 160A, and 1 for each battery) ?
In theory, I would like 1 battery to handle the max load, if the other is down for maintenance or if its BMS shuts down.
This for a 24v 3000w inverter, so max load should be 125A, or 156A after multiply by 1.25

So should I go with 125A breaker for each (will limit the max load when only one battery) , and leave the 160A as the main ?
 
Last edited:
3000W Inverter: High Frequency Inverter can do 2X Surge or 6000W, A Low Frequency Inverter can do 3X Surge 9000W
3000÷25.6=117A, 6000÷25.6=234A., 9000÷25.6=351A
* 25.6V = 3.200 Volts per cell X 8 pcs. (Using the "Nominal Power Rating" of the cells which is 50% SOC)
A 250A Breaker would be acceptable at a minimum.

TIP: On a 24V System, a 1200W Inverter Microwave uses about 75A, and a Typical Coffee Maker uses about 50A when percolating.
 
3000W Inverter: High Frequency Inverter can do 2X Surge or 6000W, A Low Frequency Inverter can do 3X Surge 9000W
3000÷25.6=117A, 6000÷25.6=234A., 9000÷25.6=351A
* 25.6V = 3.200 Volts per cell X 8 pcs. (Using the "Nominal Power Rating" of the cells which is 50% SOC)
A 250A Breaker would be acceptable at a minimum.

TIP: On a 24V System, a 1200W Inverter Microwave uses about 75A, and a Typical Coffee Maker uses about 50A when percolating.
It an HF inverter, Similar to MPP LV2424, but 3kw.
Again, I don't plan to max it out, will be running mostly resistive loads, and some lighting.
I would think that the surge would be only for a few seconds at most anyways. Would that pop the breaker immediately ? I know that breakers usually take a few seconds to open, unless you create a dead short, which will cause them to open immediately.

I also want to protect the inverter more than power heavy loads.
 
It an HF inverter, Similar to MPP LV2424, but 3kw.
Again, I don't plan to max it out, will be running mostly resistive loads, and some lighting.
I would think that the surge would be only for a few seconds at most anyways. Would that pop the breaker immediately ? I know that breakers usually take a few seconds to open, unless you create a dead short, which will cause them to open immediately.

I also want to protect the inverter more than power heavy loads.
Let me share a well known secret.
We all install a system that we think will do more than enough for out "Immediate Needs" but then next week, next month or next year we get more things that demand power OR we decide that Ohhh Today I want to use more juice to do X Thing and we facepalm realizing that we should have gone at least One Step Up. You would be very surprised to know how many people end up, upgrading within 18 months.

Breakers DO have a short delay and will not pop right away unless it is a dead short. DO NOT Cheap Out on Breakers / Fuses, many cheapo's are not up to the task, especially stuff from "No Name Vendors" from offshore.
 
Let me share a well known secret.
We all install a system that we think will do more than enough for out "Immediate Needs" but then next week, next month or next year we get more things that demand power OR we decide that Ohhh Today I want to use more juice to do X Thing and we facepalm realizing that we should have gone at least One Step Up. You would be very surprised to know how many people end up, upgrading within 18 months.

Breakers DO have a short delay and will not pop right away unless it is a dead short. DO NOT Cheap Out on Breakers / Fuses, many cheapo's are not up to the task, especially stuff from "No Name Vendors" from offshore.
I get what you are saying...
In my culture there is a well know phrase (maybe also in English) that says that "with the food comes the appetite". LOL.

That's why I went with a 3kw inverter in the first place. It's to power some critical loads that are currently under 1000w, and even then, they usually aren't all on at the same time.
Same goes for capacity, planned on just one 100Ah 24v battery, now going for two (well as mentioned, one is 105 the other will be 90Ah).

Having said that, I think that sometimes you need to start small and possibly scale up in the future. Of course you need to plan wisely how to scale up at a minimum cost / effort.
Not as easy as it sounds.

I had the urge to go 5kw with a 48v inverter, but was put off by the initial costs of both inverter and battery, without real justification in regards to my needs. If I didn't have a fiasco with a Lifepo4 order, I guess I wouldn't do two 24v packs either.
So it's always a balance between being future-proof and cost.
 
Save yourself LOT of trouble.
Build 24V/90AH & 24V/105AH battery packs and Parallel them with matching cable lengths & a FUSE on each. 15AH Difference is not significant enough to cause any issues. Over 100AH Difference and things start to get screwy... I KNOW this as I have a pair of 174AH next to 280's in one parallel bank and that 106AH difference does toss in a few curves to work around. Any more and it would be more complicated and the end result would not be what one hopes for.

One BIG Rule, is never mixing different capacity cells into One Pack, the Lowest Common Denominator will rule in such a case, so effectively your 105's would end up being no more than the 90's
HI, I have question on the cable length: On the 16s 302 battery I have. from the battery: BLue wire from battery to BMS JBD (overkill Solar) bms is 6 inches, then Black wire which is to power (-) breaker I used on Black here after black is cable 1/0 to Bus Bar. My Question was: the cable lenth for this. I don't calculate the breaker or BMS body itself right or the fuse? and how about the Busbar cable to Inverter. here I have another breaker between Inverter EG4 6500 EX from Signature Solar. to bus bar. The Breaker do not calculate in? Thanks a lot. I am new DIY.
 
Well, it's a long story but I'll try to make it short:
After an order fiasco, I'm ending up with 8 x EVE LF90K (90ah) Lifepo4 cells, and another 8 x EVE LF105 (105Ah) cells.

I have a 24v all-in-one inverter on order and want to build a battery for it.

Although I am a JBD BMS fan, this time I decided to go JK 200A BMS because of the 2A active balancing.

Anyways, my dilemma is this:
1. To build 2 separate 24v battery packs, each with its own BMS, and use them in parallel
or
2. To parallel each 90Ah cell with a 105Ah cell, and build a 2p, 8s 24v battery (total capacity of 195Ah).

Yes I know that neither are ideal, but option 2 sounds weird. The reason for option 2 is of course cost and simplicity of the build:
1. Both cell types are same dimensions (or at least very close).
2. Don't need to buy another BMS (the JK is almost $100)
3. Less balance leads, only one battery to monitor, etc...

The main goal is to make good use of both cell types. Mainly for more capacity, I don't really need more power (not planning on maxing out the 3kw inverter).

Which path would be the best in terms of cell life / cycle life ?

Or should I just give up on the idea and find something else to do with the 90Ah cells (not much that I can think of, already got some 12v Lifepo4 packs of various capacities lying around).

You thoughts and opinions are appreciated!
Both solution number 1 and 2 would work just fine. I would go with number 2 and mount the leads diagonally out of each 195AH paralleled cells
 
Both solution number 1 and 2 would work just fine. I would go with number 2 and mount the leads diagonally out of each 195AH paralleled cells
Hi, I think I screw up. At first I bought the 3x 5 kWh battery rack: lifepower from Signature solar 16s x 100 amp = 5.12 kWh each and bought 3. 3 of these are in it's own box with 3 of it own BMS from Signature Solar (batteries Rack). that is 4500 USD total... expensive... Then I found out the 302 Ah batteries cells from Docan power that Amy sold is cheaper. I bought 16s 2500 USD. Thes batteries are 16s 302 amp at 3.2 v this made it 15kWh and using Overkill Solar BMS 100 Ah 48v. I was a nob and did not buy 200 AH..
Am I mess up the batteries pack size in Ah???? all of this go to the same Busbar that supply the power to the inverter EG 6500 EX 120v 48 v system ???? Solar is 4600 wh max in summer. Winter Max at 3000 Wh, charge max at about 48 Ah after mppt show on App. Any easy example to understand about batteries pack size? I am a nob to this. thanks
 
Last edited:
Save yourself LOT of trouble.
Build 24V/90AH & 24V/105AH battery packs and Parallel them with matching cable lengths & a FUSE on each. 15AH Difference is not significant enough to cause any issues. Over 100AH Difference and things start to get screwy... I KNOW this as I have a pair of 174AH next to 280's in one parallel bank and that 106AH difference does toss in a few curves to work around. Any more and it would be more complicated and the end result would not be what one hopes for.

One BIG Rule, is never mixing different capacity cells into One Pack, the Lowest Common Denominator will rule in such a case, so effectively your 105's would end up being no more than the 90's
You left out a painful detail, two separate BMS's.
 
I decided to go with two separate packs in the end. Thanks for your inputs.

Yes, it's more components, but it gives you redundancy: One pack should be able to handle the full load of the inverter, but with two in parallel, you are reducing the load on the BMS and the cells. Also, if one fails, you still have power via the other.

I will have mine each with its own fuse and DC breaker, so that if I want to do some maintenance on one of the packs (I.e. redo a top balance) I can just disconnect that pack and everything should continue to work.
 
I decided to go with two separate packs in the end. Thanks for your inputs.

Yes, it's more components, but it gives you redundancy: One pack should be able to handle the full load of the inverter, but with two in parallel, you are reducing the load on the BMS and the cells. Also, if one fails, you still have power via the other.

I will have mine each with its own fuse and DC breaker, so that if I want to do some maintenance on one of the packs (I.e. redo a top balance) I can just disconnect that pack and everything should continue to work.
Hi
Be careful in the reconnection moment.
If you don't want to have huge current inrush, both batteries should be at the same SOC.
And this is one of the reasons why a Class-T fuse is (almost) mandatory
 
Hi
Be careful in the reconnection moment.
If you don't want to have huge current inrush, both batteries should be at the same SOC.
And this is one of the reasons why a Class-T fuse is (almost) mandatory
Yes, of course.
Sine I'm prone to forget that when actually doing it, maybe it would be a good idea to put a sticker on the breaker, with a reminder.

Thanks for the heads up!
 
Back
Top