I have a BatteryMinder 12 V trickle charger and two dead 6 V "sealed" lead acid batteries. I assume that the 6 V "sealed" batteries means that it is just an ordinary flooded battery that has been sealed shut.
Can I charge the two 6 V batteries, connected in series, using that charger?
Short answer is no. The same current will pass through both batteries to charge them. What happens when one battery is fully charged and the other isn’t quite done? The charger will keep sending the current and overcharge one battery.
If you have a constant current trickle charger, you can simply connect the 6V battery to the 12V charger and it will be fine. Do make sure to check the short circuit current of the charger so that it is below C/10 of the battery you are charging. You might not want to let the charging proceed to completion (i.e. do not top off).
For high current chargers and dead batteries, no, since it may cause reverse polarity in one of them. This is why you should not completely discharge multi cell batteries.
For high current chargers and batteries that are not completely dead, you can if you monitor voltage of each battery separately. You have to do this when they are not charging after a suitable rest time (which may be in minutes). Consider that, the end point of charging is determined not by how much current has passed, but by the final voltage of the battery. However, one battery will likely finish charging before the other one does. At that poin, it may be possible to top off the less charged one with low current.
IMHO, it may just be easier to charge each one separately, with a 7V output constant current buck converter charger for each battery, with both chargers operating from the same 12V supply.
Hooking up to 6 volt batteries in series with a 12 volt charger will work as long as you have it set on a lower output so you don't overheat the batteries.
What is a 12 volt battery? It is 6, 2 volt plates hooked up in series inside of a single polypropylene case. So a 12 volt battery is essentially six two volt batteries hooked in series and the plates all charge evenly more or less. I therefore see no reason that two 6 volt batteries each of them with three plates in them in two separate units when hooked in series as far as electricity goes it is now at 12 volt battery and it should charge in the exact same way. My father used to do this with his model boats back in the 60s and 70s when he would have three or four lower voltage wet cell batteries hooked in series to create a 12 volt and separate 6 volt battery to run his remote control boats which he built from scratch. He would also charge them with a 12 volt battery charger which could switch from 6 to 12 volts. Even though the batteries were made up of separate two volt batteries which he got from his job as a fireman (I believe they were used to power some of the radios at the time) the batteries would fully charge as if they were one physical unit and it worked fine for 20 years.
Look for the 6v-12v switch on the back of your charger. I have the same charger in the garage and it will charge 6v or 12v depending on how you switch it. If your model doesn't have that feature see below...
Make the charger work for you... Cut the voltage in half to get the 6v you need and then you can safely charge both batteries.
This below is a quote from LearningAboutElectronics.com
"To divide voltage in half, all you must do is place any 2 resistors of equal value in series and then place a jumper wire in between the resistors. At this point where the jumper wire is placed, the voltage will be one-half the value of the voltage supplying the circuit. The 12V is now 6V. VCC is split in half."
