Electrical – Charging a bank of batteries in series with a resistor

The "right" way to charge 4 batteries simultaneously would be to buy 4 small "trickle charger" style battery chargers and charge each battery independently. This would allow you to safely leave all of the batteries that aren't in active use to charge at their own ideal rate. This is the approach that your distributor is recommending to you.

It is possible, though far from ideal, to charge several 12V batteries (monoblocks with 6 cells) in parallel. It works much better if they are all the same age and they must be the same brand and same capacity. What you really want is for them to have the same specific gravity in each cell in each monoblock. It sounds like you are using flooded batteries, so checking for proper electrolyte levels often will be very important.

I'd recommend the following guidelines for charging multiple lead acid batteries in parallel:

1) Ensure that the lead lengths between the charger and each cell are close to identical. You would not, for example, want to connect the charger directly to the posts of battery A, then connect battery B to the posts of battery A, etc.) Each length of wire will have a voltage drop associated with it while passing current. You want to ensure that, under charge, the terminal voltages of all batteries (measured at the post) are very close to the same.

2) Add current limiting (like a fuse or fusable link) in each connection (both sides) to each battery. This will provide a basic (not comprehensive!) level of protection against accidental reverse polarity connection).

3) Use a good quality low rate (10-20A) charger

Your copper pipe approach satisfies requirement #1. If you use leads with inline fuses (buy fuse holders from an auto parts store and splice them inline) and ensure that the pipes are mounted in such a way that nothing metallic could reasonably short the two pipes, you've got #2.

As for why your most recent experiment didn't work, it's tough to say without more information. Wheeled automototive battery chargers typically have zero regulation - they are simply a multi-tap transformer (different charge rates) and a rectifier. They are made primarily for jump-starting/battery assist type of applications and aren't a great fit for your application. Research good quality trickle/maintenance chargers, and select one with a 10-20A output (selectable). (#3) You can then more comfortably experiment with the charge rate that best matches your application.

Always wear eye protection when working around batteries.

1. I don't think this is by definition a wasted exercise to your developing interest in battery technologies. I do think there are better ways. There are charger ICs on the market for nearly any number of cells in series. But, if it's for testing your idea, this should be fine.

2. Yes, but they will regardless of how you charge, discharge or store them. Just if you let them discharge below a certain level or charge over another level, they will degrade much faster. With an intelligent charger system, which I think your PCBs are, overcharging should be protected.

3. In charging: No, in fact, they all get charged to full. In discharging yes. This can cause increased deterioration, which then makes them get out of balance faster, which causes a snowball effect. How can you prevent that? When they are in use, make sure they are protected from over-discharge. By this mean, each cell individually. Just checking if the total gets below 7V or 8V is not going to help, you need to have something that protects each cell from going below 2.5V. Here I note that some of these charge managers I mentioned also do this for you; they are a complete battery pack protection solution.

4. You will need a set of isolated DC-DC converters. Which types are the cheapest depends on how and where you get them and where you are having them delivered, but many different manufacturers make DC-DC converters with 5V output at 100mA to over 20A. Just make sure they are fully isolated.