Haha, good. Thanks for confirmingWe Cheat!!
LOL - I put the pack together, and attach it to the Main ESS and turn it on, let the system on a sunny day bring the cells in the new DIY pack up close to full charge, the disconnect it from the ESS and balance with a bench source the last bit up to full pack voltage. Watch for any cells that might depart from the other's voltage and use an automotive bulb to pull just that cell down while the others are charging. Put on some music and work on something else in the shop for a couple hours, checking once in awhile on the new pack. When it is close, let the BMS do the rest and put it back into the ESS and let er go.
With a JK and 2a active balancing IMO there’s no point in top balance if you can hold cell voltage > 3.4 for a day, let the balancer do its job.
With such a low current (30A) it didn't make a difference. These 8 cells sit now at 3.276V exact.Connecting the charge wires to one end in such an unbalanced manner will result is extremely uneven charging.
Move one of the connections to the other end as pictured to create a more balanced current flow.
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I'm not a fan a extra bmses. Each needs the same amperage as this is a 48v 16s system so I could dive it into 2 8s or 4 4s. So they wouldn't be cheap (I'd like 350A surge current). Also every extra bms is an extra thing to go wrong... So my preference Was always a single bms. The question was really, should I divide the batteries or not. I decided against due to the fact I'd need to put the boxes right next to eachother if I wanted to maintain similar resistance between the boxes as between cells. Putting all the cells into a tower it'll take a lot less floor space. I'll simply fabricate an insulated box around my existing "fixture".Build the batteries in the size that you can move as you might need to. Use a single BMS for each assembly and connect them. By reducing the amps in each unit you may be able to buy a lower spec BMS for each but will need 2 or 3...
I liked them too, until I decided to go on with the "tower" idea.Looking for potential cases, I liked the Dewalt stacking toolbox system. That or a similar with a dolly may be right for you. Large ammo boxes look good also.
Having seen a couple teardown videos I doubt commercial battery manufacturers care about the right amount of compression force etc.It's all too much! For me at least. Much rather pay extra for a commercial battery pack and (hopefully) warranty.
Where you you read that voltage level is a proper indication of balanced SOC?With such a low current (30A) it didn't make a difference. These 8 cells sit now at 3.276V exact.
A nice feature of using a hobby charger is tgat it tells me exactly how many AH went in, but charging like this is way too slow for me.
So having made additional aluminium bus bars that let me connect all of them in parallel or in series without removing from the fixture I'm going to series charge the other 8 to the same voltage (I have a 24v regulated psu that can deliver up to 80A). I already set it up, but I'd never leave it charging like this on its own (unbalanced, with no bms). However, tomorrow I'll monitor the voltages closely and I should get able to hit the same voltage in half an hour. Then I'll equalise the cells a bit and I'll connect them in series to charge all 16 to 3.4 finally top equalising the last bit in parallel.
I'm not a fan a extra bmses. Each needs the same amperage as this is a 48v 16s system so I could dive it into 2 8s or 4 4s. So they wouldn't be cheap (I'd like 350A surge current). Also every extra bms is an extra thing to go wrong... So my preference Was always a single bms. The question was really, should I divide the batteries or not. I decided against due to the fact I'd need to put the boxes right next to eachother if I wanted to maintain similar resistance between the boxes as between cells. Putting all the cells into a tower it'll take a lot less floor space. I'll simply fabricate an insulated box around my existing "fixture".
I liked them too, until I decided to go on with the "tower" idea.
Voltage is a good rough approximation for the state of charge for any battery tech. That's basic electrochemistry. I'm not claiming LFP batteries charged to 3.276 have equal SOC. You need to charge them all the way and wait for the current to go down for this as the voltage/SOC curve is pretty flat in the middle for LFP. But showing they are still on the same voltage 24h after they have been disconnected is a useful information they are not very far off from eachother.Where you you read that voltage level is a proper indication of SOC?
Are they currently further out than when they started? (Am I "unbalancing" them?) Yes probably, but this is just a beginning. I may unbalance them more during the bulk charge, but they will be top balanced once the full state of charge is reached.It’s been proven time and time again top balance that many cell in parallel need to be cross charged.
What you are showing is actually unbalancing them. The only way you could balance them is if the tail current is held for days on end so the charge reaches that furthers cells.
And will has learned a lot since he was still wet behind the ears, he’s the first to admit everything he’s learned since day one.Voltage is a good rough approximation for the state of charge for any battery tech. That's basic electrochemistry. I'm not claiming LFP batteries charged to 3.276 have equal SOC. You need to charge them all the way and wait for the current to go down for this as the voltage/SOC curve is pretty flat in the middle for LFP. But showing they are still on the same voltage 24h after they have been disconnected is a useful information they are not very far off from eachother.
Are they currently further out than when they started? (Am I "unbalancing" them?) Yes probably, but this is just a beginning. I may unbalance them more during the bulk charge, but they will be top balanced once the full state of charge is reached.
I'm not plannig to stop at 3.276V. Cross, or no cross connection. If I'm wrong, please do share some evidence to the contrary.
Also, is Will here showing us how to "unbalance" the cells
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And will has learned a lot since he was still wet behind the ears, he’s the first to admit everything he’s learned since day one.
All I’m saying is when 30a is applied to a long string of cells such as that the charge isn’t being shared evenly. It’s easy science to understand.
The issues you are causing can be corrected but you’re basically doubling your time and effort to achieve the same thing.
I jiust bought a rack battery system with a ten year warranty and if I remember the tear down video correctly there was some form of compression.Warranties are usually for a couple of years.
I have never heard that but that is an argument for closed loop communication so the battery can reduce the charging current before there is a need to disconnect the battery.I heard these AIO chargers like to break if the battery goes away during charging.
I had to do that when I was building my camp battery. I had all the cells mounted and fully topped up and balanced, then couldn't get them out of the case for transport, so I had to put a load on them via a small inverter and work light for a few hours to get them to shrink down enough that I could get the cells out.So has anyone disassembled compressed cells later?
I mean, if I discharge them to 3.2V, can I be pretty certain no deformation results when I remove the compression? (assuming good cells etc.).
No, that was not my point. 50-60% is a good number for long term storage. The rest of my cells were fine with no swelling. My point was swelling can occur even when a cell is otherwise behaving well.So it seems 60% SOC is way too high.
I didn't mean too high for long term storage. I mean too high to disassemble a fixtured/compressed set of cells.No, that was not my point. 50-60% is a good number for long term storage. The rest of my cells were fine with no swelling. My point was swelling can occur even when a cell is otherwise behaving well.