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I have a 36V 500 watt E-Bike and 3 12V SLA batteries. I just added in series a 6V battery bringing the voltage to 42V. Adding 12 volts more would fry the regulator but adding 6V from 36V was a handy hill climbing boost.

But now I need to charge a 42V battery pack but I cannot find a SLA charger at that exact voltage, however I can find a 48 volt SLA charger but I wonder, is too much difference in voltage?

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    \$\begingroup\$ A 42V charger may be hard to find, but there is a simple solution. Just continue using your 36V charger on the original batteries, and get a 6V SLA charger for the extra battery. \$\endgroup\$ – Bruce Abbott Oct 31 '16 at 23:51
  • \$\begingroup\$ each 2V cell needs to be balanced to 2% for expected capacity and longevity , so use wisely. if all together, use temp comp. curve for deviations from 25'C for 2.3V*N cell \$\endgroup\$ – Sunnyskyguy EE75 Nov 1 '16 at 3:12
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It is very likely that you will boil the batteries and destroy all of them.

12v batteries are designed to float charge to around 13.8v, so for a nominal 48 pack this will be 55.2v. Considering that a 12v battery has 6 2v cells, each cell is only rated to 2.3v on float charge. In this setup there would be 21 cells in a a 42v pack instead of 24 in a 48v pack. This means each cell would be charged to 2.63v and start boiling off all the electrolyte.

It may be possible to find a charger which would allow you to adjust the float voltage but its not something that is likely to be listed on the specs page for the charger.

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