I have been planning to switch to one 72V charger instead of six 12V chargers. Since I am using AGM batteries, I needed some sort of way to keep the batteries balanced, since I would be charging them all in series. Since even the same part number battery can be slightly different than the one beside it, what can happen over time when series charging is that they can go out of balance. When this happens, the batteries that need less charging get overcharged and the ones that need more charging get undercharged.
Flooded lead acid batteries are easy to keep balanced. All you need to do with those is overcharge them for a while and each cell bubbles away and self balances. This overcharging would quickly murder AGM batteries like I have. Instead, a balancing system is needed. So I got to work soldering together very simple battery regulators. I got the design from an EV veteran named Lee Hart on the EV discussion list (EVDL) HERE.
Each regulator uses two zener diodes to shunt current through a resistor (light bulb) once the voltage across the battery posts reaches a predetermined level, in my case 13.0V. This way, batteries that are fully charged first will waste a bit of energy through the light bulb while less charged batteries get a chance to catch up.
Here is the finished product:
I also installed my new Kelly 7210 charger:
It's the silver square thing at the bottom. Compare this picture to the picture in last post.
It works awesome, except for the fact that it isn't actually the proper charger for these batteries. In my great haste to purchase a battery charger, I got a charger that is meant for flooded lead acid batteries (which my pack mostly consisted of at the time) with a max voltage of 88.2V (14.4V per battery, 2.4VPC). Unfortunately, AGM batteries require a lower constant charging voltage than flooded batteries.
When I charge, the bulk stage (when the batteries are at a lower SOC and are drawing full current) is fine. All voltages are even and slowly climb to 14.0V/battery and my regulators all start glowing nicely. But as soon as they go over 14.0V, a couple of batteries quickly shoot up to 15.2-15.5V (making my regulators glow very bright!), and after a while a couple of batteries go down to 13.8V. 15.5V is very bad for these particular AGMs because it can cause electrolyte bubbling at too high of a rate which may cause them to vent. Once AGMs vent, they quickly degrade because there is no way to fill them back up with electrolyte.
What I have been doing is plugging the charger through my timer so it shuts down after a certain amount of time. That way if I forget about it, they won't fry on me. I would hate to ruin my brand new pack! I finish the charge by babysitting it, watching each battery voltage and shutting it off when I know they are done.
So...I have ordered another 7210 Kelly charger - this time with a constant voltage of 84.0V (14.0V/battery, 2.33VPC) and a float voltage of 81.0V (13.5V/battery, 2.25VPC). Apparently it shipped this morning, so I should have it this week. My current one shouldn't be difficult to sell because it has the correct charging profile for charging lithiums. Anyone in the market for a 72V/10A (88.2V max) charger?
So far, this new charger is proving to be about 30% more efficient than my last charger setup. That means that my car is 30% more efficient now. Quite a significant different for one modification.