Hi all,

There is a scenario that I've been pondering, and was hoping I could get some insight from the people here.

I know it's possible, but I'm wondering what the safest and most effective way to charge multiple batteries (6V, 12V and something like 5-10 batteries) would be with a single charger. I've been reading around, and it seems that the danger comes when a battery with a shorted cell or one that is heavily discharged is connected in parallel with one that is not. From what I've seen, the battery with more charge will discharge into the weaker battery, causing "boil over" or maybe even explosion (bad!).

Could a battery isolator or battery separator help solve this problem? If so, I haven't been able to find an isolator that can isolate 5-10 batteries, only 2-4. Could these isolators potentially be configured to work with 5-10 batteries? I put together a drawing of what I'm talking about and attached it here. If this configuration is used, would it provide isolation between all of the batteries? I don't think it will; I think that with the charger off, all of the odd numbered batteries will still be connected. Could this be fixed using a solid state isolator?

I know it's a lot, but I'm trying to wrap my head around it. Any help is greatly appreciated.

 

If you attach the + lead of the charger to 10 power rectifiers you will have 10 isolated outputs. I recommend adding a fuse to each of the 10 circuits so one bad battery will not take out the entire charger.

This will put you in a constant voltage situation. Would require some resistance be added if you want to charge batteries that are mostly depleted.

 

If you attach the + lead of the charger to 10 power rectifiers you will have 10 isolated outputs. I recommend adding a fuse to each of the 10 circuits so one bad battery will not take out the entire charger.

This will put you in a constant voltage situation. Would require some resistance be added if you want to charge batteries that are mostly depleted.

True, but then the forward voltage drop of the rectifiers would cause the batteries to never reach full charge, correct?

 

Not necessarily. Are you planning to build a charger that can't provide an extra half volt? and it has no ripple voltage peaks? and it can't be adjusted? and it can't be connected to an autotransformer? and the line voltage at your house is the lowest provided for in the design process?

 

I wasn't planning on designing a charger, just a way to connect everything in a safe manner. I'm probably going to use a readily available charger that I can switch between 6 and 12V.

I'm guessing the heat sinks for this setup would have to be rather large?

 

Depends on the current. Pick a rectifier, look up its dtatsheet, look at the chart that shows forward voltage at x number of amps, figure out how many amps each battery will receive, multiply amps times the voltage drop of the rectifier.

For instance, 1 amp at .55 volts is .55 watts. If the rectifier is in a TO-220 package, no heat sink required.

 

Gotcha.

How is the heat sink size determined if the power dissipation is high? Say 5W? 10W? 30W? I'm guessing that packages like the TO-220, TO-247, etc. can handle that with adequate heat sinking?

 

It's a bit like resistors. The device has a "Theta J C". That's thermal resistance, junction to case, usually in centigrade degrees per watt.

The datasheet has that number and the highest temperature the device will survive. Hint: do not design a device to run at the edge of melting if you want it to last more than a week.

Then there is the thermal resistance of the connection from the rectifier case to the heat sink..thermal grease and such things. Then there is the heat sink thermal resistance s-a
sink to air.

Figure the watts through the thermal resistances to find temperature rise, add the room temperature and see if the rectifier is going to melt.

If it is, choose a bigger heat sink, or a fan, or work the math backwards to figure what size heat sink you need. Then go look through catalogs or thermalloy.com to find the right heat sink to buy (or fake with a piece of scrap aluminum).

 

If you insist on charging 5 or more batteries at the same time, be prepared to build your own isolator or to hire someone to build it for you. The charger will have to provide enough voltage to cover the diode loss and any other losses you build into the device. It may be time to perform a cost benefit analysis on what you propose to do.

 

Depending on the current level, talk to the guys at Perfect Switch in San Diego. When it comes to the battery isolator technology, they patented the smartest arrayed MOSFET technology around.