Batteries

Truths (?)

Concerning batteries...

A cell is a cell. A battery is two or more cells connected together in series.

A 6 volt battery is actually three 2 volt cells connected in series already. All the current produced by the 6 volt battery has to go through each individual cell, so, the current rating of the battery has to be the same current rating of each individual cell.

Identical as they may be, no two cells have EXACTLY the same voltage, so, if you connect two cells or batteries in parallel, one will always be discharging into the other, effectively overcharging one cell (or battery) while discharging the other. Process may be small to almost negligeable, but still present.

Not only common in the early 60's for tractor manufacturers to put two 6 volt batteries in series, it continued much later. I had a 1977 John Deere 4230 I bought new. It had two six volt batteries in series.

First the battery complany makes a grid out of lead. The grid is "buttered" with a special lead oxide. It is then baked for a day to adhere the oxide to the grid. NOW, what the manufactures call black magic. The grids are hung in a fiberglass vat of sulferic acid, about 480 12V batteries worth. They start to charge them in reverse polarity for so many amp hours. Then they short them out, change the polarity then put so many amp hours as needed to give you the rating. The plates are then dried and set into your battery, welded together, a top put on and shipped to you.

I designed the control system for the "dry charging process for Prestolite Battery (who makes John Deere and other brand batteries).

Which brings us to the point that the plate welding process is about the only variable besides the composition of the lead oxide and grid composition. The condition of the welding tips and the cleanliness of the welding process is key. My old company also produced the weld monitoring systems.

(snip)if you connect two cells or batteries in parallel, one will always be discharging into the other, effectively overcharging one cell (or battery) while discharging the other(end snip)

They will go into a state of equalibrium as measured from the voltage from internal resistance as Jeff pointed out.

Somethingelse is you can also serries chargers for the voltage required.

Example; My golf cart orperates at 36v so I put 3-12v chargers in serries thus ending with a 36v charging source, howerver total charging amps will only be at the highest single charger rate or 10amps in this example.

T_Bone

What makes the different voltage
between the cells is the important part
you left out...

Which is the internal resistance of the cell
itself.

If you take two identical, new batteries, charge them identically, cycle them identically, charge them identically again, and so on, let them rest and later check their voltages, there WILL be a slight difference. Maybe only a few thousandths of a volt, but, enough that if they were connected together, there will always be current flow between them.

Left tied together in parallel, (with no applied load) over time, one cell will become overcharged and the other depleted.

It's a fact.

Due to impurities, no two batteries will have the same (at rest) voltages when fully charged.

Ham radio operators have known this for years, many of them have backup batteries to stay (on the air) when power goes out. Many of them are hyper about monitoring their batteries. All of them find that sooner or later when they need battery power, they will be unhappy because one battery will have "fought" the other.

Us old-time Hams know enough to diode-block our batteries so they can't discharge into each other ;)

Red Dave is right, the voltage between cells is within a very narrow range, and in the case of parallel batteries in tractors that are used often, it probably would not matter.

The wisdom of connecting batteries in parallel has been argued over and over. Bottom line, it is done all the time, hardly ever any problem comes from it.