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I want to connect two separate batteries together and let the highest voltage one charge the lower one not exceeding the desired current. I expect it to be a solid state solution without the need for programmable parts. I was looking for something in the range 1A-10A. It does not need to be adjustable, although I would like to make a 1A one and a 10A one, for example. The voltage difference would be 4v maximum. I would like it to continue to work down to diode voltages. When balanced it would need to draw only a low current maximum. Say < 1mA. Obviously resistors would do this job. I was hoping for a solution that would maintain a higher current as the voltages converged, would be more efficient and would not get too hot. Efficiency is not a deal breaker.

The following site suggests back to back JFETs but I cannot find any more references to such a circuit and its not clear to me what the typical components I could use and the magnitude of the current when limited. http://unitedsic.com/sic-jfets/

Would it be possible to use back to back constant current source circuits made from linear voltage regulators?

The aim is to get the batteries into a "storage" state where they are nether fully charged or discharged. At the moment I am charging the fully discharged batteries from a separate supply and discharging the unused fully charged batteries to a load. I can charge a discharged battery using a charged one. However to charger boosts the voltage so it can over discharge the battery I am using as a supply. The charger does not protect against this. It would be nice just to balance all the batteries. The final voltage is likely to be within the storage range.

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    \$\begingroup\$ Maybe a ptc could solve your problem? \$\endgroup\$ – valerio_new May 1 '18 at 14:28
  • \$\begingroup\$ Would I ask too much if I'd ask what do you hope to achieve? \$\endgroup\$ – a concerned citizen May 1 '18 at 14:29
  • \$\begingroup\$ Linear regulators tend to not be very happy if you force current backwards through them. \$\endgroup\$ – Hearth May 1 '18 at 14:37
  • \$\begingroup\$ Updated with the reason. I would like to balance the battery voltages for storage. \$\endgroup\$ – BillyBag2 May 1 '18 at 14:48
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    \$\begingroup\$ How short is short and how exponential should something not be given that resistors don't have diode volt drops and will therefore transfer energy much more effectively. \$\endgroup\$ – Andy aka May 1 '18 at 15:48

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