I have now received two of the Foxtech Diamond 30AH batteries – taking 5weeks to arrive. I have been putting them through their paces to see if they live up to their spec.
They do!
I have given one of the batteries 6 charge/discharge cycles so far. Using an electronic load with a battery testing capability and voltage logging.
1/ Charging at 10A then discharging at 10A.
Resulted in 30.7AH current and 672WH power.So, their spec of 30AH and 666WH is correct.
2/ Charging at 20A and discharging at 10A.
Resulted in 30.6AH and 670WH. So no obvious capacity degredation or lesser capacity due to greater charge rate.
3/ Discharging at 30A(approx.)
Driving a constant 660mOhm load power resistor gave out 27.5AH and 590WH of power.
So the delivered power has reduced with increase in load.
The picture shows the three 2Ohm power resistors in parallel giving 220mOhm resistance. A fan was needed to keep them cool!
4/ Then, the big one! Discharging at 150Amps.
This was done using a 0.15mOhm steel wire put into a 12L bucket of water. This gives 146A at 22V.
The steel wire used was measured at 77mOhm per meter. (but increases when hot!). So 2M would be 0.154Ohm.
I measured two lengths of 4Metres and put these in parallel to produce a 154mOhm resistor.
These two coils where wound around a former as shown, then put into a plastic bucket containing 12 litres of water.
We have got 600WH of energy in the battery, that gives about 11 minutes of discharge at 150Amps
The water temperature started at 11.6 degrees. It was a cold evening!
Nervously, the switch was switched and the current flowed.
Current measured on a current clamp meter showed 151A at the start – nice!
My thermal imager was showing the cabling getting a bit hot.
The 4AWG cable through the switch was fine and barely got warm
The Diamond battery pack has a 10AWG cable from it. This is too thin for 150Amps.
After 3 minutes when the cable had built to a temperature of 160 degrees, I switched the battery off as the cable would be overheating the battery.
I quickly set up a fan to keep the wiring cool and switched back on.
The wiring from the battery stayed at about 70 degrees with the fan running.
This ran for a further 5 minutes.
Then the cable into the XT90 connector popped out! I’m not sure whether it was bad joint or the heat had melted the solder. This was the connector fitted to the battery pack by the manufacturers!
I managed to get enough contact with the cable rammed back in to continue.
I ran another 3 minutes until the battery voltage dropped to 15V.
The rise in temperature of the 12Litres of water was 52DegC- 11.6 Deg C = 40 Deg C
Using P = (4.2 X 12L x Temp)/3600, means that 560WH of energy was delivered to the water.
This is probably less than actual, given there was no insulation of the bucket and the water would have cooled slightly during the interruptions.
Unfortunately, my voltage logging was also messed up by the interruptions, so I didn’t have enough data to do a proper current and watt/hour calculation from the logs.
5/ A discharge with 8AWG cable on the battery
I stripped back the battery and replaced the 10 AWG cable with 8AWG.
I replaced the XT90 connectors with XT150 connectors.
I filled the bucket with 14L this time and again ran the pack at 150 amps.
After 3 minutes the 8AWG cable reached 70 degC. Switching the fan on reduced this to 50 degrees C.
This ran into the 150mOhm load for 14:39 minutes:seconds with no worryingly hot cabling.
The battery reached 60 deg C. The 14L of water got to 46 deg C.
The rise in temperature of the 14Litres of water was 46 DegC-11 degrees C = 35 deg C
Using P = (4.2 X 14L x Temp)/3600, Energy put into the water = 571 WHours.
From the voltage log, power = 576WHours. A remarkably close result!
And accumulated current = 28,500 mAH
6/ Charging at 20A and discharging at 10A.
One last cycle to see if the two 5C discharges have reduced the batteries capacity at all.
Resulted in 30.433AH and 666.264WH. I include the decimal points here because this is so weirdly close to the spec of these batteries. So no significant capacity degredation after the high current discharge cycles.
Conclusions.
The spec of these battery packs makes them the highest energy per weight ratio of any battery I have tested or am aware of.
At 10A discharge they have their specified capacity of 30Ah/666Watt hours.
At 30A discharge they drop to 88% capacity.
At 150A they drop to 86% capacity - 576Watt/Hours and deliver 28,500mAHours
At low currents they give 260Wh/KG which looks to me to be a record breaker for a lithium battery!!
Even the mighty tesla cells only come in at 250WH/Kg
Note: I’m afraid the OpenPPG battery comparison speadsheet OpenPPG Battery Comparison - Google Tabellen has some wrong capacity claims for some of the batteries.The NCR18650 cell = 224WH/KG not 259WH/Kg.
The next best cell I have tested is the Sony VTC6 which comes in at 225WH/Kg at light loads, 210WH/KG at full load. The Molicell P42A is also a contender having a slight edge at 230WH/Kg according to the data sheet- but I haven’t tested it at working current - yet!
The 10AWG cable and the XT90 connector from the battery are not fit for purpose.
Changing to an 8AWG cable with a XT150 connector is adequate as long as there is some cooling taking place. 4AWG or two 8AWG cables in parallel would be even better.
If you want the best batteries, don’t mind paying and don’t mind ripping these batteries apart to change the cabling, it looks like these ones have the edge!
Maybe I should invest in 4 more to make a really powerful six-pack!!