Battery Myths and Tips

adas wrote: »

Aloha, could you verify or comment on the battery charge workings?

Talking about 24v 875AH Heavy duty Electric Forklift batteries

1: Does a weak cell, say 2.01 V bring down the whole other 11 cells?

• Lead or frame short would (at lower currents anyways) make your 12 cell battery look like an 11 cell battery... If in parallel with the other batteries--most likely this battery would "take all of the charge" and overcharge the other 11 cells. When load is applied, the other batteries would supply the load until they are near dead, then the 11 overcharged cells would start to supply current. The shorted cell would probably run very hot (temperature wise).

• High resistance--more or less, like the battery is not even in the chain.

• Contaminated acid, more Sulfates than other cells--would probably be, more or less, like the "open cell"... Little current flow when charging, quick loss of voltage when discharging.

• Batteries in other wise identical condition (temperature, amount of use/aging, equal resistance, chemical activity, etc.) would be very close to each other in overall voltage... The difference in charging would be the resistance in the charging path(s). Batteries in a higher resistance path would get (and deliver) less current. Batteries with a lower resistance path would get (and deliver) more current.
• In electrical design--to get circuits to share well--you would design the distribution network to have a higher resistance (and control it so that each path matched resistance) vs the low resistance energy source/sink (i.e., battery). So, if all of the cables are the same length (and not too heavy gauge), and they all go to the same point (or low resistance bus bar)--then the batteries should receive equal current (and slight differences in battery resistance, chemistry, temperature, etc.) will not have a large effect on the current flow (because the distribution cables have higher resistance--they effectively regulate current sharing).
• If, however the cables have very low resistance vs the battery resistance, or the cables have different lengths/gauges/etc., then the current sharing to the parallel battery strings is much more difficult to guarantee (minor differences in each battery would affect the charge current distribution)...
• With parallel batteries--the battery with the lowest current path and the "best chemistry" will tend to source/sink the most current--and will tend to wear out the quickest over time. Which is why it is not recommended to parallel old and new batteries together--the new batteries would tend to cycle most of the load and wear out quickly to match the older batteries in the parallel set.

• See above... The more batteries you have in parallel, the more difficult it is to guarantee current sharing... Especially if the batteries are of different ages and there is not a balanced electrical distribution system.
• My personal experience is that I would avoid large numbers of parallel strings and tend towards higher voltage series strings... The absolute number--I don't know--but somebody recommended here about 3 parallel strings maximum--and that would probably be my choice too...
• To check how far off your matching of distribution is... Place the batteries under heavy load (or even heavy charge) and use a calibrated DVM and measure the voltage drops on the cables and the voltage at each battery terminal...
• From the Battery FAQ here, the difference between a 90% charge and a 100% charge for a 24 volt bank is 200mV... So, I would probably be aiming at a 20mV maximum differential on a battery terminal to battery terminal difference (that is an accuracy better than 1000:1).

• Don't know---but there have yet to be any battery additive claims that have been supported by any battery manufacturers (or large users like telephone companies).
• I read one guy/place that used some sort of caustic to convert a dead lead acid battery to an "Alkaline" Lead Acid battery (rinsed out acid electrolyte and replaced with alkaline electrolyte"... You can read about the process here--I stumbled on the website last week. I have no idea if this is safe or works at all, or if the poster is full of hot air--use at your own risk.

• For an AGM type battery--they are very close to 100% efficient in Coulomb charge... 100% of the electrons out equals 100% of the electrons back in charging (may be 98%?). The amount of energy delivered vs required to charge is different... The battery's voltage is lower when delivering current, and higher when charging--so, roughly, you end up around ~90% energy efficiency...
• For wet cell Lead Acid batteries--because of the losses from equalization (and other chemical differences?) the overall energy efficiency is around 80%--at least those are the rough numbers we throw around here when sizing systems. I am sure there are more accurate numbers (based on a whole bunch of variables) if we look around (or ask Jim/Crewzer).