Batteries
I built a spreadsheet a while ago comparing cost per KWh of different kinds of batteries. This doesn’t tell the whole story as it doesn’t include life cycle. A car starter battery probably only has a few hundred cycles in it whereas a long life LIFEPO4 one will last 10,000 if treated right. But that can be a very how long is a piece of string question which is why I have not included it as its down to personal behaviour
The spreadsheet is a bit out of date now as some lithium options have come down in price. I will update in due course (sorry very UK focussed) also due to the current crisis some prices may have increased (I will try to de-UK this a little, add prices in euros and dollars and code it by country?)
There is a silly saying that ‘friends don’t let friends buy used batteries’. This definitely makes sense for lead batteries unless you’re buying them at scrap value. That said if you can find a battery pack from a forklift that has been well maintained and not abused it should be able to provide many more years of service. That’s what I’m currently using and its been going strong on the farm for 7 years of its 17 year life. A well cared for forklift pack can last a very long time.
Battery Options
Lead
I built a spreadsheet a while ago comparing cost per KWh of different kinds of batteries. This doesn’t tell the whole story as it doesn’t include life cycle. A car starter battery probably only has a few hundred cycles in it whereas a long life LIFEPO4 one will last 10,000 if treated right. But that can be a very how long is a piece of string question which is why I have not included it as its down to personal behaviour
The spreadsheet is a bit out of date now as some lithium options have come down in price. I will update in due course (sorry very UK focussed) also due to the current crisis some prices may have increased (I will try to de-UK this a little, add prices in euros and dollars and code it by country?)
There is a silly saying that ‘friends don’t let friends buy used batteries’. This definitely makes sense for lead batteries unless you’re buying them at scrap value. That said if you can find a battery pack from a forklift that has been well maintained and not abused it should be able to provide many more years of service. That’s what I’m currently using and its been going strong on the farm for 7 years of its 17 year life. A well cared for forklift pack can last a very long time.
Battery Options
Lead
Lead batteries are roughly 80% efficient so throw away 1/5th
of what you charge them with
You want Deep Cycle only. Anything that says marine or leisure will be useless. A starter battery even worse.
Starter batteries are designed with a very fine honeycomb structure with huge surface area to allow a lot of ion exchange in a hurry and produce a big jolt of amps. If deep cycled this damages the structure which eventually just sheds from the plates building up at the bottom of the battery and eventually shorting the cells out.
Deep cycle batteries have thicker solid plates. So are physically more robust and can stand a lot more ablative wear before they become useless. Often they will have a bigger gap at the bottom of the battery so that as material sheds it can build up for longer before shorting the cells.
Plante style cells are quite an extreme example of this.
You want Deep Cycle only. Anything that says marine or leisure will be useless. A starter battery even worse.
Starter batteries are designed with a very fine honeycomb structure with huge surface area to allow a lot of ion exchange in a hurry and produce a big jolt of amps. If deep cycled this damages the structure which eventually just sheds from the plates building up at the bottom of the battery and eventually shorting the cells out.
Deep cycle batteries have thicker solid plates. So are physically more robust and can stand a lot more ablative wear before they become useless. Often they will have a bigger gap at the bottom of the battery so that as material sheds it can build up for longer before shorting the cells.
Plante style cells are quite an extreme example of this.
Lead comes in two main flavours. AGM or Flooded. Gel are available too but not really suitable for this application as they do not tolerate overcharging and die quickly.
AGM tend to be more expensive but low maintenance. They don’t have quite the same output capacity either. Very useful if they need to be left unattended.
Flooded tend to be cheaper and longer lasting. Provided you keep them topped up with deionised water and equalise them.
Terms like T-105 or L-16 denote a form factor of battery. Lots of different manufacturers will make batteries to these form factors.
A very popular starter set up used to be 4x Trojan t-105s in series to make a fairly decent sized 24v 220ah bank. Handy for a cabin or workshop shed and would provide about 2kwh if useful power which goes a long way unless you’re running electric heating.
Traction cells come in a various sizes up to several thousand Ah Capacity in 2v individual cells. These would still make a very robust and long lasting battery back for less upfront cost than lithium. Especially if not used aggressively and deep discharged.
Lead batteries need a minimum of C/10 charging current to prevent sulphation and early death.
So if you have a 24v 500ah battery pack. You will need at least 50amps of charging. Or 1200w of solar (roughly)
There are many flavours of lithium option but I’ll stick to a couple. They are mostly 90-95% efficient in terms of charge/discharge cycle
Laptop cells. Aka 18650s. These have become very popular with youtubers and the like and people with far too much time on their hands. You can very cheaply assemble a reasonable sized battery pack out of used laptop cells by buying or acquiring old laptop battery packs. Stripping them apart testing the cells and reassembling them with either premade snap together plastic parts or by spot welding them.
I have seen people making rudimentary spot welders with a momentary switch. A car battery and a big solenoid hooked up to the welding electrodes. I’ve tried this technique myself but kept burning my fingers.
It is not a good idea to solder 18650s unless you used a very large soldering iron for very short amount of time. Sustained high heat will kill them.
LIPO RC plane packs.
Not a very good idea! These can provide a tremendous amount of current for fairly low cost. But the chemistry is pretty volatile. If you discharge them too deep by accident they are instantly dead. If you try to recharge them from this state they WILL catch on fire. Very handy for a high power ebike as they can put out many 10s of times their capacity in a short time. For instance some are rated at 100C aka a 5ah pack could put out 500amps very briefly. Conversely a lead acid battery you will be lucky to get C/5 of the Ah capacity.
LG CHEM can be purchased as stand alone units of recycled from crashed Nissan leafs.
BYD another car manufacturer that makes stand alone units with everything you need built in.
Tesla. Used tesla packs are available on ebay. They are not too expensive and quite handy because they are natively just under 24v. So a couple in series will make a pretty respectable 48v battery. Tesla also offer the power wall.
LIFEPO4 You can buy these either as pouch cells intended for use for ebikes or as large prismatic cells. The prismatic cells come in 3.2v increments in sizes up to 500ah
The main manufacturers or brands of LIFEPO4 prismatics are CALB, WINSTON and SINOPOLY. Not really sure about the difference between brands. There is a stockist of them in the Czech republic who are pretty reputable if you don’t want to import direct from china.
The come with bolt on terminals only with no cell protection so you need to provide or build your own BMS.
https://www.ev-power.eu/Sinopoly-40Ah-300Ah/
Other Batteries
There are a couple of other technologies.
Nickle Iron are very popular with a fringe crowd who think they are the best thing since sliced bread. They have a few advantages in that they use an Alkaline electrolyte which is not as nasty as battery acid and they batteries themselves will last for ever. However their self discharge is very high and they will run themselves flat unless kept under constant charge. They are also very inefficient (Around 65%) So waste a lot of power as heat.
Aquion salt batteries. I don’t know much about these. Low discharge capacity but seem interesting.
Flow batteries. These are very interesting and work by mixing different electrolytes. I suspect we’ll see these used in huge grid scale operations but not so much for home use
Conclusion
If I was starting again from scratch and possessed the skills I have now or greater I would try to get a used EV pack from a wreck. Or use prismatic cells. A decent traction battery is still worth a go but Lithium has the advantage that it just wants to absorb electrons much more readily. Its more efficient and can be discharged much deeper without damaging the battery, and can be kept at a middling state of charge without damage.
Lead needs to be kept fully charged to prevent sulphation. Can’t be discharged below 50% if you want them to last.
One of the issues that lithium can have is it doesn’t like the cold. So if kept indoors this isn’t really a problem. But charging a lithium batteries should not be charged when cold (see individual battery data sheets for what this means)
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