Charging several 10S4P 36v battery packs with solar?

[wrybread]

I've got three lithium battery packs built for electric bikes, each a different capacity, from 300 to 500 watt hours. I'm thinking about using them as a battery bank in my RV since my lead acid batteries are about 6 years old now.

I've got a few questions for the lithium battery wizards:

- I'm guessing I can't put all 3 battery packs in parallel, since they're likely to be at different voltages. Correct?

- If each battery pack needs to be separate, I was considering using something like this to charge each one. Does that sound adequate? Or is there something better to use?

https://www.amazon.com/Numerical-Regulator-10V-120V-Converter-Adjustable/dp/B01GFVI6R6

- What's the max current that I should be charging the packs with?

- For battery longevity, should I use a lower voltage than the full 42 volts? From reading here, is 41 volts the recommendation?

- Any suggestion for a watt meter that I can put on each battery pack that would tally the watt hours in and out? The one I have only shows watt hours in one direction.

- I've seen some mention of 3 phase charging for lithium batteries. I know lead acid uses 3 phases (bulk, absorption, float), does lithium need something similar, or is a constant voltage sufficient?

Thanks for any help.

 

[john61ct]

Danger Will Robinson!

I would never use any LI chemistry but LFP or LTO for House usage in a mobile living space.

Boom bad. . .

 

[wrybread]

I should mention, I have a good place for the batteries, in the metal box that formerly housed a generator. It's very well insulated and vented to the outside. If there happened to be a fire it would go out the side of the camper, not in the living space. I might add some Hardie Board as extra insulation.

 

[john61ct]

Still.

Maybe with an automatic heat-triggered catapult that would throw it 30+ feet away. . .

 

[wrybread]

Yup, I understand the risks.

Still wondering about the logistics of charging these through solar though, whether I use them in this RV or a different project.

I've also got a sort of taco truck (actually a bar built onto a trailer) that I need to make a power pack for, and a fire there would be downright entertaining, ha.

 

[John in CR]

No, don't charge to 4.2V per cell. In order to give the 8-10yr warranty on the batteries of electric cars, companies like Chevy and Ford charge to only slighly above 4V/cell. There are some test results here in the battery section where it was demonstrated that charging to less than 4.2V/cell paid big dividends in terms of battery degradation. Both the upper and lower ends of the max voltage range were shown to have a significantly greater effect than the current levels as long as using too high. I've done it with my cell phone too, ie never below 30% capacity and I try to limit the top end at less than 90% full, and my battery seems to have a much longer life.

Another thing I like is that it gives me a big safety margin running without a BMS. My bulk charge cutoff is at 4.1V per cell, so a cell would have to get out of balance by more than 0.1V before any could exceed 4.2V. 4 of my bikes have automotive grade batteries, 2 in use for 3 years and 2 in use for 2 years. One of the packs is withing a range of +/- 0/0.015V, and the other 3 are all within +/- 0.01V and that range has been consistent whether fresh off the charger, at mid voltage, or toward the bottom of the voltage range of my use, so I've never had to balance any of the packs. If you have a pack with a bms that balances as 4.2V/cell, then it may be a good to fully charge the pack occasionally.

If you get your 10s packs all to a similar voltage +/- 1V, then you can safely parallel them. Paralleling all the 10s packs is the way to go, however your biggest issue will be running your system at that voltage unless it is already set up to run three 12V lead acid batteries in series.

 

[wrybread]

Thanks for that.

however your biggest issue will be running your system at that voltage unless it is already set up to run three 12V lead acid

Both the RV and "taco truck" are wired for 12 volt, but it's all lower power stuff (stereo and lights mostly), so I was thinking about using something like this to step the batteries down to 12 volt:

https://www.amazon.com/gp/product/B0756T983Q

And a 36 volt inverter like this:

https://www.amazon.com/dp/B0080YV0SY

The AC doesn't need to power much, mostly just charging laptops.

 

[john61ct]

Laptops use DC to charge. Just match the V, A, pin size and polarity.

All the charging intelligence is built into the computer.

 

[999zip999]

Buy some of those LTO for 12v 6 of them or high quality lifepo4 and forget about it. Ohhave some sense wiresto monitor so don't forget get about it. Quality cells need little maintenance But you should know what's end is up. Use your ebike batteries for carrying you UP. Or for the end of the world. Or don't stress your bike batteries.

 

[wrybread]

Laptops use DC to charge. Just match the V, A, pin size and polarity.

I've done that in the past (no need to match the A by the way, except as a minimum, as long as the voltage is matched), but it sounds much better in theory than practice in my experience. There's a lot to be said for being able to plug a laptop into any power plug in the camper, as opposed to having to use one or two hard wired charging cables. Not to mention other gizmos that need to be plugged in from time to time.

I prefer to go overboard on solar panels (currently have 900 watts) so I don't need to squeeze every watt.

 

[Hwy89]

The Victron Energy smart solar charge controllers have adjustable max and min voltage so that they can be used to charge lithium packs. I have two retired bike batteries reconfigured into 6 s 32 p and I have the controller set to charge to 24V.
I have a 30 A 24 to 12 volt step down converter and a 24 volt inverter. There is a extra cable and plug directly from the battery pack for plugging in the inverter when needed. I don't leave it hooked up because I've had problems in the past with inverters draining the batteries even when switched off.
One cool feature of the Victron charge controller is that it communicates with my cell phone via blue tooth so any time I want to check the charge I can open the app on my phone. It also keeps a usage history.
It has been in use since May and so far so good. The only glitch was that they did a software upgrade while I was parked many miles from a cell phone signal so I had to manually check the batteries until I got back to civilization and could update the software.

 

[jonyjoe303]

if all the batteries are the same 10s, you can connect them together (parallel) as long as you connect all the balance cables together also so they equalize. It doesnt matter if the packs are different capacities. Just make sure they all have the same voltage when you connect together. When connected together you will have one 10s12p pack. For instance my 4s40p 220ah pack is actually 4x 4s10p packs connected together in parallel.

If you plan to use solar you need a bms that can handle solar without creating voltage surges, for your situation you need something like the chargery bms16t (you can find on ebay about 110 dollars). It uses external relays to stop the charging (by disconnecting the solar panel). It works on packs up to 16s, very programmable on cell cutoffs parameters. It has built-in lcd screen to program, no need to use a smartphone to connect to it.

For measuring amps in/out get the tk15 coulometer (also on ebay about 23 dollars), its very accurate I use on all my lithium batteries.

Bulk/absorption is the same thing, you can set to 41 volts (if you want cells to only charge to 4.1 volts), and set float even lower to 40 volts or less. Once it hits float it will charge at very low amps. On my battery when it hits float its charging at 1 amp or less.



tk15


overvoltage protection relay. For solar I use one of these relays as a deadman switch, in case charge controller fails and bms fails. It measures voltage directly from the battery and will disconnect the solar panel (with relay) if the voltage gets too high. For instance if the max voltage you don't want exceeded is 42 volts, you program it to pull relay at that voltage. Its cheap insurance (less then 10 dollars for the protection relay). Picture of my system I actually have 2 relays between the solar panel and controller (1 chargery bms/ 1 over protection relay). On this picture it shows a 0-40 volt protection relay, but you can find ones that read from 0-99 volts. You always want to disconnect the panel to stop charging, if you you disconnect the controller from battery that causes problems when the controller gets disconnected from the battery while controller still connected to solar panel.

 

[flippy]

these projects are always fun to do.

few questions:

do you also use or want to use a AC inverter?

do you have the abillity to reconfigure packs? aka: weld or solder the packs

 

[wrybread]

Man, thanks jonyjoe, phenomenal info. Much to digest! I see a lot of eBay in my future.

And I'm confused, in that setup, what exactly is the charge controller?

And what amperage do you send to your batts during bulk/absorption?

 

[john61ct]

The solar controller regulates the charge cycle. A HVC or other separate device that isolates the pack from the source once Full could be considered another part.

Note many SCs require a battery to remain connected as long as the panels are, which then needs to be accommodated in the design.

The charge current rate is best kept under 0.4-0.6C for longevity. But over 1C is common when fast charging is needed.

PS batteries pull current, the source does not "send" or push, just makes current available.