DIY EV Part 21 - When buying batteries is Expensive BUY MORE!!!

[@ThisRandomUsername]

I'm sure you'll already know this, but if you decide to make a powerwall-style battery for use with solar panels, you'll want to oversize the pack to your consumption by around 30%. Lithium ion wants to remain between 20% and 80% for more cycles. LiFePO4 is much longer lived and usually pretty cheap even compared to second-hand car batteries. Again I'm sure you know this. Also well done for leaving 13:26 in there. Edit: Solid copper is usually more desirable for fixed wiring because it takes less physical space. It's also better at conducting heat, meaning lower resistance on the inner strands. Probably not much of a consideration here. Stranded copper is just more flexible and resistant to vibrations.

[@alibro7512]

To be honest I haven't done any research yet on the powerwall so thank you for the points. It might make more sense to keep the VW batteries for other projects but I'll keep my options open. While editing I laughed so much I couldn't resist leaving it in. 😂 I'm sure I've heard that current mostly flows on the outside 'skin' of a conductor therefore having stranded cables is better for high current. Whether fact or fiction VW seem happy to use solid conductors so I'm sure they did plenty of testing to confirm it was OK.

[@ThisRandomUsername]

@@alibro7512 Depending on your use case these batteries could be great for a backup system, but will have reduced capacity in a few years if cycled constantly. LiFePO4 can do thousands of cycles where Li ion can do hundreds. Li ion is good for backup because it can do high C rates, meaning you need fewer amp hours to do the same job. Yes, skin effect is a problem with higher frequency AC. I've worked with RFID card readers and we tested Litz wire to good effect at 125kHz, but the extra expense didn't make it worth it. In a DC conductor skin effect isn't a problem.

[@ISpiksterI]

Does this battery pack case have any venting valve, umbrella valve or something else?

[@alibro7512]

To be honest I don't know as I didn't really look for one. I would presume it must have something to relieve pressure from heating and cooling batteries but the top of the case is some kind of fibre glass so it will have a degree of stretch and flex. The big take away I had was it has no manual service disconnect. It is relying 100% on the contactors not failing closed.

[@JamieJones1985]

How come you didn't just reuse the VW bms?

[@alibro7512]

I'm not aware of anyone having cracked it yet. I believe Simp BMS can read the data from the BMS but cannot make it balance the pack. At least not yet. If anyone does crack it I still have the BMS modules and I know how they were connected so I could refit them. In the meantime I'm still working on getting the Nissan Leaf BMS to work with this pack.

[@JamieJones1985]

@@alibro7512 Mine are staying pretty in balance at the moment. I'm going to work on getting balancing working when my second pack arrives, I've now got a can log from a GTE on charge so should be able to trick the pack in to thinking it's on charge, and I'll just charge one cell a little higher than the rest.

[@alibro7512]

@@JamieJones1985 I tried draining one of the cells a little before stripping down the pack and with everything still connected nothing had changed when I came back to it the next day. I guess it needs a poke in the right place to make it balance. Do you still have the VW BMS on your pack and if so do you think it's working?

[@JamieJones1985]

@@alibro7512 im using the slaves, but not the master. The active balance isnt happening. They stay within 30 to 60 mV usually 30 ish, until the cells get up over 4v then they drift closer to 60mV delta. New pack ahould be here tomorrow, and not in a massive rush to mount it so hopefully get the balancing figured out. I'm also interested in the HV isolation measurement.

[@alibro7512]

@@JamieJones1985 It sounds like your cells are different to mine, In my eGolf pack there are 8 masters and the rest are slaves. The masters are linked by CAN cables. Are you having issues with the insulation or just concerns of what could happen?

[@proinnsiasbreathnach6656]

Where are you getting eGolf packs at that kind of price - or am I just looking in the wrong places :) Fair play though - it's projects like this give us a glimpse into the future

[@alibro7512]

I got them off evbreakers.com who are local to me.