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.


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.
http://liionbms.com/pdf/bostonpower/swing5300.pdfFirst 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 ?

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.



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.

Next day was full sun

, 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.

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

, 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.

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%.

Watch this space
..........