Idiot's guide to a self build battery

[Caesium]

Has the variance between your worst cell when first identified and the average across the remaining cells fallen? I'd be hoping to see whatever the gap is close a bit over time. Glad to read it's working well

Yep, they're all back within 1mV now, 3.319 on most, the weakest cell is actually lower at 3.318. I think its not unusual that the variations expose themselves at extreme levels of charge. Once they start getting discharged again they sync back up pretty quickly.

I expect if I discharged the pack all the way down to 0% SOC then that same cell would fall away down to 2.5v faster than the others - but I barely discharge below 50% at the moment. Which I'm also happy with as it prolongs the life of the pack

[Joeboy]

Has the variance between your worst cell when first identified and the average across the remaining cells fallen? I'd be hoping to see whatever the gap is close a bit over time. Glad to read it's working well

Yep, they're all back within 1mV now, 3.319 on most, the weakest cell is actually lower at 3.318. I think its not unusual that the variations expose themselves at extreme levels of charge. Once they start getting discharged again they sync back up pretty quickly.

I expect if I discharged the pack all the way down to 0% SOC then that same cell would fall away down to 2.5v faster than the others - but I barely discharge below 50% at the moment. Which I'm also happy with as it prolongs the life of the pack

Excellent! I find the idea of battery storage and what it can bring to be rather mindblowing. I am away to Turkey on Tuesday and will be back at start Nov. I have asked for the batteries to be delivered then but son will be here in any case. Did you go back to Pylontech setting on inverter? I am looking forward to seeing the soc drop to super sllooowww charge/discharge.

[Caesium]

Haven't gone back to Pylontech setting on inverter yet, I could try that tonight.. be interesting to see if its any different now they've had a few days balancing but actually I think its a bit inconsequential for me because I don't want to rely on having Pylontechs, so if it can work without then so much the better.

I have a vague plan to replace all my Pylontechs with LF280s (another set of 15 would more than do it) then use the Daly BMS I've ordered to monitor it all over RS485.

If I wanted to get really fancy I could write some scripts to pretend to be a Pylontech BMS over CANBUS and control the inverter using the data from the Daly. Mostly I'm concerned with shutting off the charge amps if any one cell gets too high and the BMS can't keep up with shedding the voltage, as currently I have little protection for that.. kind of trusting that the current BMS is enough.

I think the price of these 280s are going up on alibaba though, my previous supplier who did them for $110 each now seems to be asking £130 :/

[Joeboy]

That£s off the back if this thread.

[Caesium]

Just did a bit of fun maths. Peak rate on my Go Faster 40.75p. Offpeak 8.25p. So 32.5p saved per kWh that I can timeshift rather than use directly.

Ok there's some losses involved so I have to use a bit more offpeak than I would have done. I'll be generous to that and take it down to 25p/kWh saved.

I get through anywhere from 11 - 14kWh a day, which these LF280s + my Pylons happily satisfy with ease. It would be very close to emptying them every day with no Pylons.

So on average, 25p * 12.5kWh = £3.12 a day, £1170 a year.. I reckon I'm about two grand in on these, so payback in two years! Not bad.. Just in time for the energy guarantee to expire and see where we end up with price differentials then..

[nowty]

So on average, 25p * 12.5kWh = £3.12 a day, £1170 a year.. I reckon I'm about two grand in on these, so payback in two years! Not bad.. Just in time for the energy guarantee to expire and see where we end up with price differentials then..

That’s nuts ain’t it.

[Caesium]

Just posting before I forget, Joeboy you might find https://www.gobelpower.com/lifepo4_decoder.html handy when your cells arrive, type in the long number along the top and you can see exactly when your cells were made

For example one of mine is 04QCB76847000JC2J0008386. Scanning the QR code on the top gives you the same thing but its so small its very fiddly. Seems my cells were made 18th Feb 2022

[Joeboy]

Just posting before I forget, Joeboy you might find https://www.gobelpower.com/lifepo4_decoder.html handy when your cells arrive, type in the long number along the top and you can see exactly when your cells were made

For example one of mine is 04QCB76847000JC2J0008386. Scanning the QR code on the top gives you the same thing but its so small its very fiddly. Seems my cells were made 18th Feb 2022

Thank you Caesium. Who ever knew? (Apart from you of course). Thanks again.

[Caesium]

Today's micro-project; adding a power switch to the balancer board. 5 seconds soldering:



I've added this for a couple of reasons - it (mostly) avoids a spark when plugging it as no current flows until I turn it on, and secondly I occasionally want to be able to easily turn it off for a few hours so I intentionally let any weak cells wander off-voltage so I can identify them. With the balancer permanently on and no real monitoring of it thus far, I realised that measuring individual cell voltages occasionally wouldn't tell me much, as this balancer will be working to keep them all the same anyway.

If I turn the balancer off for a few hours (days?) then I can keep an eye on cell voltages and any that do drift significantly may need further attention.

[Caesium]

todo list..

• order 16 LF280K and assorted bits-n-bobs
• build 15S DIY stack to sit parallel to Pylontechs
• sell 3/6 Pylontech to fund this habit
• order 16 more LF280K
• build a second 15S stack in paralle
• sell remaining Pylontechs
• reconfigure both DIY stacks to be 16S
• spend years messing with smart BMS systems

yes I ordered more The Pylontechs sold for silly money, more than I paid for them. £900 a pop, so £5400 total potentially once they're all sold, which I'm replacing with £4000 worth of LF280Ks. That takes me from 14.4kWh to around 28kWh and puts a grand back in my pocket. Though some of that is being eaten by balancers and odds-n-sods.

Once I have no Pylontechs left I'm planning to rebuild both stacks to 16S, as I'll have no other 15S to be in parallel with. I'm not envisioning any problems with this. The LXP inverter reckons it can supply charge up to 60v and by that point I'll have far more capacity than I know what to do with (with no panels yet and only 5h of 3.6kW charge capability, I can only refill 18kWh of it a night anyway) so I'll set the charge voltage quite low, probably 54.4v (3.4*16). This should give the cells a nice easy life and not max then out to 100% every night. I can tweak up the voltage marginally if I ever do empty them.