I see why you want a voltage doubler. I tried a setup like that a while ago, but with using a PIC instead of a comparator to control the cap dump voltage.
What I found is that as the battery voltage raises, it doesn't really need to be hit with double the voltage. For example, when the battery gets to 15v you don't need to hit it with a 30v pulse. Lead acid batteries seem to prefer the difference in voltage to reduce as they charge, but also increase the pulse frequency.
This is exactly what the fixed reference voltage comparator does. I usually set my reference voltage at about 25v. When the battery starts to charge it gets a good 12 to 13v over the battery pulse about 7 pulses/second (depending on what I use as the input). Then during the middle part of the charge when the battery is at around 13.5v it's getting solid 11.5v pulses over the battery at about 10 pulses/sec. Then to finish charging at around 15v it gets about 10v over the battery pulses at around 14 pulses/sec.
The last part provides a good desulfation pulse train that doesn't heat the battery or over-gas it too much.
FETs work pretty well to dump the caps, just watch that they don't heat up too much. I use a 3" fan mounted on a heatsink for mine. A BJT transistor works fine in place of a gate driver. You should have no trouble adapting the 555/scr circuit to a FET. The IRFP260 is pretty robust and will handle a fair bit of power. I looked through the thread for a schematic but couldn't find what I was looking for. There's one by Ron Chase and another from RS_, maybe they will re-post it for you or someone else has better searching skills than me
John K.
What I found is that as the battery voltage raises, it doesn't really need to be hit with double the voltage. For example, when the battery gets to 15v you don't need to hit it with a 30v pulse. Lead acid batteries seem to prefer the difference in voltage to reduce as they charge, but also increase the pulse frequency.
This is exactly what the fixed reference voltage comparator does. I usually set my reference voltage at about 25v. When the battery starts to charge it gets a good 12 to 13v over the battery pulse about 7 pulses/second (depending on what I use as the input). Then during the middle part of the charge when the battery is at around 13.5v it's getting solid 11.5v pulses over the battery at about 10 pulses/sec. Then to finish charging at around 15v it gets about 10v over the battery pulses at around 14 pulses/sec.
The last part provides a good desulfation pulse train that doesn't heat the battery or over-gas it too much.
FETs work pretty well to dump the caps, just watch that they don't heat up too much. I use a 3" fan mounted on a heatsink for mine. A BJT transistor works fine in place of a gate driver. You should have no trouble adapting the 555/scr circuit to a FET. The IRFP260 is pretty robust and will handle a fair bit of power. I looked through the thread for a schematic but couldn't find what I was looking for. There's one by Ron Chase and another from RS_, maybe they will re-post it for you or someone else has better searching skills than me
John K.
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