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Author Topic: Mixing Lithium and Lead  (Read 386 times)
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« on: February 13, 2018, 11:23:36 PM »

Last year I bought several cheap second hand Growatts with 5 kWh batteries off fleebay, one of which I had been keeping as a spare. But my experience is once you buy a spare item, it never gets used so I started thinking of uses for it. The battery cannot be used without the Growatt controller as its operation is locked down. I was a bit bored the other day so decided to take the back off the battery and have a closer look to see if it could be easily unlocked. tumble

It contains 14 battery modules in series and a proprietary protection circuit board. Each battery module contains 18 of 5,300 mAh cells in parallel so a total of 252 cells. Each cell is apparently two 18650s in parallel in a single package. Shocked

First thing was to rip out the protection board and then the two battery racks conveniently come out to give 24v each. Then I had a crazy idea, my Sunny Island runs at 24v using forklift cells. Would it be possible to add them in parallel to increase my OffGrid capacity ? wackoold

It sounded like an absurd idea, mix lithium and lead ?, but the more I thought about it, the more it started to be a possibility. Charge and discharge graphs are different, but my operating range of lead is typically between 100% and 75% SOC. Voltage wise, this matches favourably with lithium between 100% and 40% SOC. Although I would only be adding circa 20% more Ah capacity, the actual usable capacity might increase by 50% or more. At high SOC, the lithium ones would provide most of the power so may extend the life of my forklift cells which are now 6 years old.

I took out the two lithium racks and connected them in parallel with my forklift cells when the voltages were roughly equal to prevent any initial current surges. fingers crossed!

Then I reduced the charging voltage of my Sunny Island from the current 30.5v to 29.4v and set the temp compensation to zero. 29.4v gives the max 4.2v charging voltage per lithium module and is still good enough for the forklift cells. bike

Next day was full sun chocolateteapot, so I measured the voltage of each lithium module regularly. Voltage increased more slowly than just with lead alone and the lithium was taking 75% of the current. When the voltage rose to around 28v the current was shared equally, then over 29v the lead was taking most of the current. The voltage of each lithium module was identical except for one with was 0.1v more, so cells looks nicely balanced so far. When the charging day was over at 29v, the lithium then discharged slightly and continued to charge the lead until the voltages stabilised at around 27v so continued the absorption charge of the lead even after the sun went down. Grin

On discharge, from 27v down to 26v the lithium was supplying almost all the load.
From 26v down to 25v, the lead started to supply some of the load.
At 25v lithium and lead were supplying exactly half and then the voltage hardly moved down to 24.5v by the morning.

My first result is encouraging, no dramas sh*tfan, definite increase in capacity, better voltage stabilisation on discharge and batteries accepted the max charge from the Sunny Island all the time without throttling back. This is an advantage I had not even thought of. ralph

Going forward I need to sort out some sort of protection circuit for the lithium as I cannot risk leaving it day after day charging the lithium to near 100%. norfolk

Watch this space     ..........

10kW of PV.
Half ton of lead battery storage.
Quarter ton of lithium battery storage.
200 litre hot water storage.
Two storage heaters.
Water harvesting from underground river.
A few cold frames and raised beds.
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« Reply #1 on: February 13, 2018, 11:40:32 PM »

There's hundreds on fleabay

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« Reply #2 on: February 14, 2018, 02:07:27 AM »

if you search for BMS it will give you a lot of options

there's a hell of a lot to chose from... most will just monitor the cells and dump power into resistors (making heat) to discharge any cells which are over discharging

but... you can get fancy ones which will dump power from high cells into low cells

if you parallel up the balance wires, then you'll only need one BMS for the whole lot :-)
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« Reply #3 on: February 14, 2018, 09:08:18 AM »


Now I know what the inside of my battery looks like!!!

I can see it wouldn't be that difficult to re-wire things internally to allow the cells to be charged by an external power supply. As I may have noted elsewhere simply connecting the positive and negative leads from the Growatt battery to a charger does nothing as the circuitry obviously needs a signal from the Growatt SP2000 unit.

I some very heavy duty (Albright) DPDT solenoids which can be used for this.



16 x BP380 on a Lorentz tracker/ SMA SB2500, 16 x Chinese 80W/ SMA SB1700, 16 x BP380/ SMA SB2500. CTC GSi12 heat pump/ Gledhill ASL0085 300litre EHS/ 3off Navitron 4720AL solar ET panels and an Immersun T1060/ T1070/ T1090. 7 x Tianwei 235W/ SMA SB1700/ Growatt SP2000.
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An unpaid Navitron volunteer who lives off-grid.

« Reply #4 on: February 14, 2018, 09:15:10 AM »

    Exciting stuff Nowty,  I wonder what it would take to put together a 120volt bank, ?
   Could I chain a series of Growatts together, ?
    I guess the thing to do would be to study the suitable BMSs first and then try and find the way to keep them happy.
 I could sell my Lead acid friends.. genuflect
« Last Edit: February 14, 2018, 09:29:01 AM by biff » Logged

An unpaid Navitron volunteer,who has been living off-grid,powered by wind and solar,each year better than the last one.
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« Reply #5 on: February 14, 2018, 09:48:25 AM »

Assuming the SI is set for charging LA batts  SI will trickle charge as is normal for LA,  so isn't there a danger of overcharging the Li batteries during trickle charging?

I would be thinking about protecting against fault current flowing between banks  by adding a fuse between them?

Would be nice to get a schematic of the Growatt BMS, if the 'authorisation' handshake could be faked the BMS would probably still operate.


4kwp south facing array  SMA 4000TL grid connected.  2x30tube Navitron solar thermal panels (east/west). Arada 5kw S/C WBS. 25000Ltr underground rain water tank. KTM E-Bike  Cool
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« Reply #6 on: February 14, 2018, 10:04:10 AM »

Would it not make more sense to connect the pack in parallel with one of your existing growatt units, even if at the module level?

If you're looking for a cheap but useful BMS, this one is worth a look:

The majority of the ones on eBay have no adjustability on their settings so you can't vary the over voltage and under voltage limits.

Was there any fusing between the parallel packs in each module out of interest?

If you held onto the existing BMS it might be of interest to someone with a bare Growatt and other batteries.
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