Single string or...

D

Dmcbudman

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Single string or multiple strings?

I currently have my 18650 setup split into separate strings. a string of roughly 200watt packs, one of 150w and a small string of 85w. Each with it's own cheap bms and tied together on the negative and positive rails.

Not sure if it's proper, but I know many do this.

Atm I'm finally going thru and testing each pack, testing the capacity (only checked individual cells previously).
I'm doing this because my packs go a little out of balance after a few cycles, maybe .07v at most.

I only charge them to 4.1v (if they get that high) and my bms doesn't do any balancing there.

So now I am waiting for this to get delivered
https://www.ebay.com/itm/3-16S-48V-...o4-Lipo-Li-ion-Battery-Equalizer/153423625416
And I'm considering how to use it.

Thinking about just swapping it around to each string as needed. But it would be so much simpler if I just wired the whole thing into one string and left the balancer connected at all times.

Anyone have any advice what you think is best?
 
I Guess that One also only balance when above 4.2v. it uses some smaller chip and resistor to burn off the energy.

With that Said n+1 strings are Always Good so you can do maintenance
 
Seriously?
Figured active meant always active, not just active after it reaches capacity.
Sigh lol
Yeah I have each pack of 20 cells hot swappable, with a spare for each string :)
 
I doubt it is active. Looks like a big resistor for burning energy. But the ad doesnt state much and couldnt bother to Google today :)
 
Sorry :) better now than when you have rebuilt the system
 
Dmcbudman said:
Single string or multiple strings?

I currently have my 18650 setup split into separate strings. a string of roughly 200watt packs, one of 150w and a small string of 85w. Each with it's own cheap bms and tied together on the negative and positive rails.

Not sure if it's proper, but I know many do this.

Atm I'm finally going thru and testing each pack, testing the capacity (only checked individual cells previously).
I'm doing this because my packs go a little out of balance after a few cycles, maybe .07v at most.

I only charge them to 4.1v (if they get that high) and my bms doesn't do any balancing there.

So now I am waiting for this to get delivered
https://www.ebay.com/itm/3-16S-48V-...o4-Lipo-Li-ion-Battery-Equalizer/153423625416
And I'm considering how to use it.

Thinking about just swapping it around to each string as needed. But it would be so much simpler if I just wired the whole thing into one string and left the balancer connected at all times.

Anyone have any advice what you think is best?
Click to expand...

The Problem - and the solution to it - lies in the misinterpretation you do in the bold marked issue.
I do the same thing - several strings plus not using full voltage swing - as you do.

The balancer WILL do balancing once a cell got out of sync more than a higher voltage difference as usual. It is correct that the string will be quite out of sync at that moment, but the balancer prevents it from getting worse, and the protection function will work anyway.

You can einther live with the faked loss of capacity (being out of sync), you can load to 4.2 every now and then for quite awhile (with-rebalances the pack if you give it time enough) or you can check your pack every now and then - or all three on your choice.

just my 2c.
 
I should add, that i have found (here)
https://secondlifestorage.com/t-Good-Chinese-BMS-review

a BMS which is totally programmable via Bluetooth or PC, which gives you adjustment of Over/undervolatge Limit, Balancer start voltage, plus choosing if it balances only during Charge(current flowing) or "static" (all times). It comes for about 30 Bucks (10s version). This is moderately high for your string sizes, but you should just make them alittle bit bigger then it matches quite nicely. As far as i have tested it until now (with a special constructed test Pack with different voltages plus SD cell) it does everything as intended.
Disadvantage is that the App works, but it has a gruesome user interface, and the operation manual is even worse.

I am close to write something better, given here is interest enough or that company would honour it somehow to me.

Btw, just yesterday i ordered 2 more, since i have modified my approach for the mobile home "enhancer" i desrcibed here
https://secondlifestorage.com/t-Project-capacity-enhancer-for-mobile-home

for not having a common BMS for five packs in parallel, but a separate one for each pack (of 36 Volt 18 Ah), only a little bigger than your ones, just for safety reasons. (less wires, less wire lengths, less solder points, BMS directly attached per Block)
 
Hmm yeah I guess that balancer isn't a total loss. I'll feel much safer occasionally top balancing them with 1.5a doing the equalization, rather than a few milliamps.

I do something similar as you. I dump my solar into lead acid and draw from that for my liion. Except I only use the liion for my inverter and dc loads.
Prolly be more efficient just running the lead and I'm sure I'm losing a lot converting up and down. but I'm keeping the powerwall lol, plus thats the safest way to do things with what I have atm.
 
If your only intention is to active (I understand by active balancing transferring energy from upper cells to lower cells and not burning energy from upper cells. It does not have to be on at all times either.) balancing a LI-Ion battery then consider this Bluetooth Balancer.

I received mine last week and have been testing it for two days now. Works like a charm, not than my battery needed balancing at all but it is doing its thing. App is very easy and user friendly, two languages availables, English and Chinise.
 
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