PV Solar - what the view in the UK in 2021?

[Spacey73]

You have been moving in similar circles to me it seems

Worst thing you can do to an ebike or any other battery is to stick a top floating BMS on them. To balance it you have to leave the battery at 100% which causes tendril growth and that's how they catch fire especially lipo's. The balancing amount on the normal ebike BMS are only 0.25w a cell and would take weeks to balance.... running them to empty causes them to go out of balance very quickly and the BMS has no hope of keeping up.

The best device is one that balances at any voltage and just shunts the current around at 18w a cell ...72 x faster and much kinder on the pack than your regular BMS. Also means you can charge them to 80% without worrying about balancing as they are always in balance.

Lipo packs like yours rock and I did have them in my Violator ebike (20kw motor) but fire risk was too high for me. I went with 30A 18650 cells 20 series and 10 parallel so 1.8kwh pack that could potentially push out 21.6 KW of power for 10 secs.... like you say if pulsed it's not a problem. Building it and cost was a mare 8 years ago though.

If you want to get into solar don't get them fitted, do it yourself and have them in your garden not on the roof. Easy to angle them into the sun and they can be propped up against the fence. cheap if you get them from old new stock in bulk.

I'll never sell back the grid though...5p/kwh no thanks that 1 kwh can make me £3 mining Ethereum a much better way to spend the surplus energy. Solar without a battery is kindof pointless unless you work from home or are retired. I will get a Tesla powerwall at some point though.

 

[Glan gluaisne]

Both my electric motorcycle and boat have been running on LiFePO4 cell packs for a couple of years now, mainly because of niggling concerns about a lot of the LiCoO2 cells around. If NMC cells were available at a reasonable price, without having to either connect up hundreds of small cells, or pay a premium for bigger cells, I'd probably have opted for them.

I think we may well have "met" virtually before, perhaps on EndlessSphere? (I used to be a moderator on there, years ago), or maybe one of the other ebike forums.

Top balancing is OK if carefully managed, and if the termination voltage is conservatively chosen, but it's inefficient and time consuming, and also means charging to 100% SoC every now and again. I built a few top balancing BMS units,before designing a flying capacitor BMS, which is not only virtually loss free, but also balances at pretty much any SoC.

I built a bit over 6 kWp of PV into the roof of our house when I designed and built it, south facing, and the panels saved the cost of slates, as they are the roof covering, so overall it turned out to be cost effective, especially as the FiT was still a thing when we built this place, so the system paid for itself a year or so ago, and so the ~£1k/year income is double our annual electricity bill (for an all electric house, electric heating, water heating, etc), giving us a negative energy cost for the house, including charging the car.

 

[Spacey73]

I lived on Endless Sphere

Flying Capacitor BMS is the only BMS I will rely on to balance a battery pack. Lifepo4 we couldn't get them to catch fire no matter what we ried, shot them with shotgun, crosbow, over charged them ....err...set fire to them. A123 pouch cells from dodgy chinese source on Alibaba.

I think £3.50 for a 100Whr 18650 low ah but high drain (1.5ah Samsung cells) new each is a steal. Had to drive to the other end of the country and pile them in the Model S.

What is the spec of your boat? I want to build an RC powered hanglider just so I can glide above the cornfields, should be doable with stripping out the 2 stroke engine and slapping in a few Turnigy RC motors in it and a ton of Lipo for 15 mins flight.

Here are my last two creations....

 

[mpandrew]

Worst thing you can do to an ebike or any other battery is to stick a top floating BMS on them. To balance it you have to leave the battery at 100% which causes tendril growth and that's how they catch fire especially lipo's.

Think you mean dendrite growth, maybe you're mixing gardening with batteries? There is an argument for not trying to balance packs and just accept the loss of capacity in each cycle versus wasting energy trying to get all the cells to precisely the same voltage. This is particularly relevant for storage systems where you don't want to deeply discharge your packs - trying to bring this somewhat back to the original thread conversation!

If anyone is interested, I have 50 x A123 AMP20M1HD-A new cells available. I say new, they are 2015 vintage but never been touched and all still showing correct delivery voltage. That amounts to 3kWh of capacity for some enterprising soul to stick together in a pack. If it's something you'd like to discuss, please send me a private message. I'll put them on Buy/Sell in a few days.

 

[Spacey73]

Ha, terminology all got mixed up...you're quite right dendrite's.

I think the best way with larger packs is to just charge every cell up to a set max once a month to balance. I will have a separate balance cable port in case I need to manually do it.

Really simple for Li ion pack, just have a single 5 amp DC supply set to 4.1 volts or slightly lower and when the battery pack is nearly 75% full or so just charge each of the cells series packs to 4.1.

Balance should last all month without a normal balancer in place if you are discharging to 20% and not charging past 8%.

I really think a solar system without a battery pack is not worth it.

 

[Glan gluaisne]

One of the ways I used to use to fast charge and balance packs years ago was to use individual cell chargers. I brought out every cell connection to a big multi-pin connector and used a load of high power isolated DC-DC converters, run from a big 48 V supply, to charge every cell group to about 4.15 V. The DC-DC converters could be set to an accurate terminal voltage and had current limiting, so there was no risk of ever overcharging a cell in the pack. The snag with this system was that I still needed a BMS on the pack to handle the over-discharge protection requirement, but that was made a lot simpler by just sensing the voltage of each cell group and initially reducing the controller power if any cell group approached the lower threshold voltage, then cut the power to the controller if any cell group reached cut-off.

TBH, I've never had a pack get down that low, as I always use a home made "fuel gauge" to monitor how much energy had been taken from the pack since the last charge. That was set to be a bit conservative, so I've always had loads of warning of charge level getting a bit low.