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A couple of weeks ago I found the batteries in my M915 dead. I had been out of town for a couple of months and the last time I started it was back in August. I checked battery voltages and the 2 batteries that are in parallel for the 12 volt tap were dead only showing a couple of volts each. The second set that are in series off the first 2 for 24 volts were not.
One of the problems us MV owners have when we need to recharge batteries, is there are usually 4 of them and most of us don’t have four 12 volt battery chargers. Since you ideally what to charge each battery individually for equal charging, it becomes a time consuming process.
A year or so ago I built a capacitive charger. I’m not going into detail on how it works, you can Google it and come up with all the information you need. In simple terms, it is a constant current charger that uses the value of the capacitors in the circuit to determine the charge current. There is a rectifier in the circuit to convert the AC line voltage to DC voltage. An added benefit is the pulsing DC output is good for desulfating batteries and I have had good success using this charger to rejuvenate NiCad rechargeable power tools batteries in addition to flooded lead acid type batteries. You can’t use it on lithium batteries.
The capacitive charger can charge any number of batteries in series where the total voltage is less than the 120 volt line voltage. There is a voltage gain above line voltage in the rectification from AC to DC, but to keep it simple I just use 120 volts as my rule of thumb maximum (I’ve never charged more than a 48 volt pack anyway).
I hooked all 4 batteries up in series and started charging them. You don’t have to worry about 1 battery getting more current than the others since the current is the same for all when connected in series. The charger I built will charge at 1, 4 and 7 amps. Since 2 of the batteries were not dead and I didn’t know how fast they would come up, I charged them all at 4 amps. You have to check to determine when they are approximately 80% charged. You do this by checking individual battery voltage. Once you start going over 15 volts while charging, you are getting into the equalization point and the battery will start actively bubbling. I don’t like to boil the water out even though it’s easy to replace, so I normally reduce the charge rate at this point.
Since the 2 batteries that were not dead were over 15 volts within a couple of hours I took them out of the circuit as each went over 15 volts. The other 2 needed longer so I set the timer to charge them 12 hours. When I checked in the morning, they were still a little low. I charged them for another 6 hours and they started going over 15 volts, so I let them equalize a little in case there was any sulfating from them being dead.
Once you reach the point where the battery(s) starts bubbling you are about 80% charged. When the 2 that were dead reached that point, I connected all 4 back in series and set my charger on 1 amp and let them charge 12 hours. The next day I checked the voltages after sitting for 6 hours after charging and they were 12.85, 12.82, 12.79, and 12.77. The spec sheet for these say a new battery should be 12.85. These are almost two years old so not too bad.
This type of charger is very handy being able to charge multiple voltages and is cheap and easy to build. But, a word of caution if you decide to build this type of charger, when the charger is on with no load (not connected to batteries) there is AC LINE VOLTAGE ON THE CHARGE LEADS! Safety first! I never have mine plugged in when it is not in use or I’m connecting or disconnecting it. If you are charging the batteries in the vehicle, disconnect the vehicle battery leads first.
One of the problems us MV owners have when we need to recharge batteries, is there are usually 4 of them and most of us don’t have four 12 volt battery chargers. Since you ideally what to charge each battery individually for equal charging, it becomes a time consuming process.
A year or so ago I built a capacitive charger. I’m not going into detail on how it works, you can Google it and come up with all the information you need. In simple terms, it is a constant current charger that uses the value of the capacitors in the circuit to determine the charge current. There is a rectifier in the circuit to convert the AC line voltage to DC voltage. An added benefit is the pulsing DC output is good for desulfating batteries and I have had good success using this charger to rejuvenate NiCad rechargeable power tools batteries in addition to flooded lead acid type batteries. You can’t use it on lithium batteries.
The capacitive charger can charge any number of batteries in series where the total voltage is less than the 120 volt line voltage. There is a voltage gain above line voltage in the rectification from AC to DC, but to keep it simple I just use 120 volts as my rule of thumb maximum (I’ve never charged more than a 48 volt pack anyway).
I hooked all 4 batteries up in series and started charging them. You don’t have to worry about 1 battery getting more current than the others since the current is the same for all when connected in series. The charger I built will charge at 1, 4 and 7 amps. Since 2 of the batteries were not dead and I didn’t know how fast they would come up, I charged them all at 4 amps. You have to check to determine when they are approximately 80% charged. You do this by checking individual battery voltage. Once you start going over 15 volts while charging, you are getting into the equalization point and the battery will start actively bubbling. I don’t like to boil the water out even though it’s easy to replace, so I normally reduce the charge rate at this point.
Since the 2 batteries that were not dead were over 15 volts within a couple of hours I took them out of the circuit as each went over 15 volts. The other 2 needed longer so I set the timer to charge them 12 hours. When I checked in the morning, they were still a little low. I charged them for another 6 hours and they started going over 15 volts, so I let them equalize a little in case there was any sulfating from them being dead.
Once you reach the point where the battery(s) starts bubbling you are about 80% charged. When the 2 that were dead reached that point, I connected all 4 back in series and set my charger on 1 amp and let them charge 12 hours. The next day I checked the voltages after sitting for 6 hours after charging and they were 12.85, 12.82, 12.79, and 12.77. The spec sheet for these say a new battery should be 12.85. These are almost two years old so not too bad.
This type of charger is very handy being able to charge multiple voltages and is cheap and easy to build. But, a word of caution if you decide to build this type of charger, when the charger is on with no load (not connected to batteries) there is AC LINE VOLTAGE ON THE CHARGE LEADS! Safety first! I never have mine plugged in when it is not in use or I’m connecting or disconnecting it. If you are charging the batteries in the vehicle, disconnect the vehicle battery leads first.
