Battery charging, series strings, parallel

[BruceM]

In researching engineering articles and battery charging techniques for my own 120VDC battery bank, I found that long series strings of batteries, even well matched, are notorious for problems using a typical series charger.  The early electric car folks did most of the pioneering work in this area, as they were often using strings well over 150 volts. They ruined a lot of batteries so that we don't have to, if we pay attention to their work.

For a serial string, over time, some batteries get overcharged, and some chronically undercharged.  The latter get sulfate build up and loose capacity, the overcharged batteries eat away one plate and fail.  Equalizing by protracted, slow overcharging can reduce the problem, but the better performing batteries still suffer some plate erosion during equalization. (Which is also a big waste of energy.)

The reason is because of variation in materials and manufacturing- each battery has slightly different characteristic charge and discharge impedance.  Without very frequent equalization, the difference in state of charge of the fastest charging batteries vs the slowest becomes enough to destroy both.

The solution is to use shunt regulators on every battery.  Manzanitamicro.com is one company that did some pioneering work in this approach. By adding a simple shunt regulator to each battery, and reducing charge current once one of the battery shunt regulators signals that it is shunting current, fast charging batteries are saved, and the current is shunted to the adjacent slower batteries (not entirely wasted as heat).  The shunt method of serial charging is also very valuable for AGM batteries, which can't be equalization charged heavily without shortening their life.  (Heavy service batteries with heavy plates can take a lot of equalization charging.)

With an intelligent charge controller, you can monitor and log the time delay between batteries reaching charge, so as to identify batteries which may need attention. (Desulfation, cycling, or replacement).

I built my own linear battery shunt regulator which is temperature compensated (probe on each battery).  Temperature compensation is critical, as colder temperature charging requires a much higher cyclic charge voltage. When I see chargers with fixed voltages, I cringe.  The regulators have bussed opto-isolated controls to select float, cyclic, or equalization charging (two signals).  Each regulator has an opto-isolated output that is an analog signal proportional to the shunt current. (one analog signal per battery)

While I am still working on the smart, datalogging controller, I have used this system manually on my 10 string battery and am very impressed. In just a few discharge/charge cycles, the disparity in charging times between batteries reduces- to less than a minute on year old (crap marine) batteries.  Equalization is not needed on a regular basis, though I will add a very brief equalization period after peak charge daily if PV or Wind power permit.

Hopefully new 24 and 48V charge controllers in the future will incorporate this approach. Without it, very high quality batteries (well matched impedance for charge and discharge) and lots of equalization are needed.

Parallel batteries have their own serious problems with charging, but there is no practical way correct the problem through the charger. It can only be mitigated by battery matching (same age, condition, charge/discharge impedance) and frequent equalization charging. The practice of paralleling a bunch of small batteries has a well documented, poor outcome.

Hopefully someday soon we'll see the shunt regulation scheme of series batteries being applied to the 48 and (later) higher voltage battery bank commercial charge controllers.  

[Jedon]

  Cool! I love the mad scientist stuff. Some boiling beakers would fit right in! What do you do with 120V DC? Can you rectify right to AC and use it as house power? I have a 14KVa UPS that has 10 12V AGM in a 120V DC configuration and a couple hundred pound transformer.

[BruceM]

My new off-grid home has 120VDC lighting, plus some 12VDC, no AC at all in the home. 

120VDC is handy- currents are low, wire small, long runs to solar panels and from batteries are practical.  Some computer gear switching power supplies can handle DC at 120V, some have to have it boosted to 150VDC or so.

Edison had it right for DC, his systems were all 110VDC. A good compromise for safety and practical wiring/distace.  For lighting efficiency, I opted 120V, I hope Tom will forgive me.

 

[rcavictim]

Bruce,

That is very impressive work and your first post explained the problems and shunt regulator solution very well.  Please continue to keep us all updated with you progress.