battery charging 2 6 volts wired in series

I have an 806 international (only had it about 6 months) Haven't had any problems with the battery before. Tried to start it today when it was a bit cold (about 40 degrees here in Montana today) cranked for a while and ran the battery down.
It has 2 6 volt batteries in series. I disconnected them and charged them one at a time, but was wondering if I'd have messed anything up if I would have left them connected and charged as a 12 volt system??

dmiller: You wouldn't have messed anything up, you can either charge them as two 6 volt batteries or as one 12 volt in series. Charging as two 6 volt may have told you something that you couldn't learn had you charged it in series. Did one battery require a lot more charging than the other? You could have a defective battery, (the one that took substancially more charge) and it will eventually take the good battery down. If you caught it in time, you might stress test the poorer battery, if bad enough, replace it.

There is also the argument on two battery systems, that one should always replace both at same time. I will say this, I've had good results changing one battery in a two-six, series system, however not in a two-twelve parallel system.

If you found they took roughly same charging, you may as well keep using them as long as you can tolerate them.

Sure, you can charge the series combination which equals 12 volts with a 12 volt charger. After all thats what happens if you charge a single 12 volt battery as it has 6 cells in series versus only three in a 6 volt battery.

However, Hugh raised some good points for you. If EITHER battery is defective, so is the series sum of BOTH, so charging separately may provide a clue of such as well as a hydromoter check of all cells in both batteries. Look for proper and similar electrolye levels above the plates in all cells of both batteries and see if any are always low or exhibit a gray or milky off color indicative of a bad cell.

If the levels look good and equal with no off colors and theres no evidence of a battery problem, Id just start with a series charge at 12 volts n see if that cures it n not worry much for now. However if problems persist then Id try seperate charging to see what happesn and look for differences in charge rates and times and levels etc

I agree with Hughes good observations for whatever thats worth. That and a buck may buy a cup of coffee UNLESS at a Yuppy Starbucks which I or most farmer type gents dont frequent anyway.

Ol John T and all

I replaced the 2 6V batteries in my 1650 Oliver diesel with a HD 12V battery and have not had any starting problems for years. It is eaiser to maintain 1 battery than 2. I dont have any IH tractors that are new enough for the 12V systems.

We charged 8 12 volt batteries connected in series/parallel for 24 volts for cold room tests. We would check all the battery cells using a hydometer before installing in a vehicle.
Then we would cold soak the vehicle for 24 hours at -50 F. Once we were at this temp we would pull the batteries at this temp & let them thaw for 24 hours before charging. This was on an M60 Tank. They weren't good at starting at that temp. We did a Soviet tank to -65F and used the same batteries for 3 starts. It was their preheat system that made the easy starts. Hal
PS: Here's a hydrometer like we used. They gave me this when I retired.

John: Getting rather eloquent for someone who signs off Ol John T.

I was an engineer and farmer n farm machinery jockey for years, then that whole law school thing got me more into "words" after all what else do they have to sell lol Im still Ol John T to friends n neighbors