Battery Balancing Time – and ‘Why Thundersky?’ Someone Asks…

A few people are asking me why I have chosen Thundersky Lithiums over conventional lead-acid deep cycle batteries. Firstly, lithium phosphates are lighter than deep-cycles, and Lithium polymers are even lighter again, approximately half the weight of the phosphates, making them useful for a variety of applications other than just electric vehicles, such as electric helicopters for example (yes, these are being developed as fully-electric one and two-person carriers).

Cycle Life

Another important reason is cycle life. Lithium phosphates have a much longer cycle life than lead acids. They’re more efficient too, delivering most of their amp-hour capacity at a high discharge rate. You can only get around half the delivery from lead acid batteries which typically exhibits a Peukert’s Law characteristic where discharging at a higher rate removes more power from the battery than basically calculated. For example, a 200 amp-hour battery at 40Ah rating may provide 10 amps for 40 hours but cannot supply 20 amps for 20 hours, as the time remaining would essentially result in less than the expected 20 hours (oh, note that lead acids are generally based on a 20Ah rate).

There’s lots of stuff on the net about Peukert’s law so I’ll save you getting bogged down here. Suffice to say that the exponent of this law varies for different batteries; a typical deep cycle Peukert exponent is 1.3. Thundersky has stated less than 1.08.

When looked after, Lithiums in my Starion should last for years….

Lithium phosphates are more sensitive than lead acids and require careful charging and discharging. This is not to say you can’t put a BMS on a pack of lead acid traction batteries – I’m sure they would benefit too – but for lithium phosphates they are essential, as you can see below.

Battery Balancing Time

Last weekend we were balancing our batteries prior to installation, lining them up and connecting in series for charging with my new Zivan. The voltages checked out okay; a couple of batteries came in around 3.6 volts and the rest close to 4.2 volts. Balance resistors are used to protect some batteries. Why is this so? Simply because not every battery that is manufactured is 100% identical, due to varying production tolerances, uneven temperature characteristics and so on.

Over time some batteries can become stressed and fail before their expected time. A cell could become charged before the rest are charged and then it is overcharged, shortening its life. Batteries are balanced when all cells in the traction pack have the same matched voltage per cell when fully charged or discharged.

And with battery management, setting the high and low charge limits is important – the idea is the make sure that when the lithium phosphates are charging, they are not over charged, and lithiums also don’t like to go flat either, as their integrity can literally fall apart. For my Thunderskys, we need to keep them above 3.2 volts.

Differences of Opinion

There’s always contention on the best life-enduring method for lithiums, whether to keep them fully charged or partially charged and if rapid charging is ultimately more damaging. General consensus is to charge more frequently and leave as partial, albeit near ‘full’ charging, more often.

When a typical laptop lithium ion battery gets old, resistance inside builds over time due to oxidation, depending on the type of battery compound inside (be it cobalt (LiCoO2), manganese (LiMn2O4) etc). The lithiums I have, Lithium Iron Phosphate (LiFePO4 or LFP for short) are less volatile, have a greater power density and are not susceptible to overheating (though efficiency may be impeded if they do get hot).

Breakin’ In Is Not So Hard to Do

There has also been discussion amongst EVers about how new LFPs should be treated, that is, ‘broken in’ to a charging regime. Some have mentioned that Thunderskies can fail prematurely if they are discharged below 35% too early.

Breaking in one’s batteries is like ‘breaking in a new car’, i.e., driving carefully for a short while and not thrashing the engine. In the lithium battery case, it has been suggested to not fully deep cycle past 30 initial charging cycles to below the suggested 35% capacity. Ultimately I plan not to fully deep cycle them too often anyway, where possible, to prolong life.

Tagged as: Batteries, battery, electric vehicle, ev, lithium, Starion, Thundersky