Since we have all been on house arrest for the past few weeks due to this virus crap, I put my time to some good use rebuilding a used Makita charger and modifying a battery for individual cell charging and balancing
Most balance circuits use a passive system in which a resistor is used to bleed off excess charge until the low cells are able to catch up. This is a very slow process that also waste energy and creates undesirable heat
My setup is an active balance design where it will change each cell independent of the others. If one is full then it doesn't charge it and if another is dead then it will get the full charge current. The battery pack was rebuilt with the dead cells replaced with other used cells so invariably it will not stay balanced that way. Also the original battery packs from Makita have a design flaw where the BMS runs off of only one cell draining it down while not in use for extended time. Apparently this has been fixed in the newer ones but I still have a handful of the old style.
Also, the standard Makita charger, if it detects three failed attempts to charge or errors it will lockout the battery from charging. By modifying the batteries and using them on this charger I can get many more years of use out of them
In order to make this work, I needed to use separate charging boards/circuits for each cell. The are connected to each other in series and to each cell. To prevent them from back feeding and shorting out, which it normally would in this setup, I had to use separate isolated power supplies. So there is a separate power supply and charger for each cell. I have it set right now to bulk charge at 3 amps but could up that to 5 amps if needed. I figured based on the data sheet for these cells 3 amps should charge it quite fast without causing much cell heating.
I also added extra cell holders on the outside so I can charge some 18650s for my flashlight, etc.
Installed on the front is a charge indicator for each cell so you know when each cell if full and you can see if any are starting to fail or have issues.
Here are some photos of the interior of my modified charger.
So far I only have two charge cycles on it through one battery pack. I will modify more packs as they give me trouble or I run across other thrown out ones.
PS: yes it can charge two batteries at a time. The current will be split between each pack so 1.5 amps each or 3 amps for one. The only issue I see with this is if one put a dead pack on while a fully charged pack was already in. A lot of current may black flow from the charged into the dead pack. Would have to test this and see how much current it really is
Most balance circuits use a passive system in which a resistor is used to bleed off excess charge until the low cells are able to catch up. This is a very slow process that also waste energy and creates undesirable heat
My setup is an active balance design where it will change each cell independent of the others. If one is full then it doesn't charge it and if another is dead then it will get the full charge current. The battery pack was rebuilt with the dead cells replaced with other used cells so invariably it will not stay balanced that way. Also the original battery packs from Makita have a design flaw where the BMS runs off of only one cell draining it down while not in use for extended time. Apparently this has been fixed in the newer ones but I still have a handful of the old style.
Also, the standard Makita charger, if it detects three failed attempts to charge or errors it will lockout the battery from charging. By modifying the batteries and using them on this charger I can get many more years of use out of them
In order to make this work, I needed to use separate charging boards/circuits for each cell. The are connected to each other in series and to each cell. To prevent them from back feeding and shorting out, which it normally would in this setup, I had to use separate isolated power supplies. So there is a separate power supply and charger for each cell. I have it set right now to bulk charge at 3 amps but could up that to 5 amps if needed. I figured based on the data sheet for these cells 3 amps should charge it quite fast without causing much cell heating.
I also added extra cell holders on the outside so I can charge some 18650s for my flashlight, etc.
Installed on the front is a charge indicator for each cell so you know when each cell if full and you can see if any are starting to fail or have issues.
Here are some photos of the interior of my modified charger.
So far I only have two charge cycles on it through one battery pack. I will modify more packs as they give me trouble or I run across other thrown out ones.
PS: yes it can charge two batteries at a time. The current will be split between each pack so 1.5 amps each or 3 amps for one. The only issue I see with this is if one put a dead pack on while a fully charged pack was already in. A lot of current may black flow from the charged into the dead pack. Would have to test this and see how much current it really is
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