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I'm constructing a power supply based on solar cells backed by batteries. The solar cells deliver between 0 and 4 A, depending on the time of day, while the charger I plan on using, the LT3652, has a maximum charging current of 2 A. I've started to consider running two of the chargers in parallel in order to utilize the full 4 A, but I'm very uncertain about how they will behave drawing on the same current source.

Does anyone have any hunches, ideas or opinions regarding this setup? A rough sketch of what I'm talking about can be seen below (note: this is not meant as a circuit diagram, just a sketch of what I mean -- don't let the fact that it's drawn using circuit symbols confuse you, it's more like a flowchart).Circuit sketch

Sorry about the vagueness/scope of the question, but I don't have much experience with charging circuits and want to avoid doing something stupid.

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  • \$\begingroup\$ Have you heard of the Free Charge Controller project? freechargecontroller.org \$\endgroup\$ – davidcary Feb 26 '11 at 20:30
  • \$\begingroup\$ @davidcary: That looks like a wonderful project. Thanks for informing me. However, they seem to focus on charging 12 V batteries, and from what I can tell, the firmware is still incomplete. \$\endgroup\$ – gspr Mar 3 '11 at 14:51
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    \$\begingroup\$ Here is an interesting document on charging batteries. See around page 15. \$\endgroup\$ – user4796 Jun 26 '11 at 14:09
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Short answer: Putting power supplies in parallel is rarely a good idea.

Why?

Because the power supplies will have slightly different output voltages, one will supply more, and one less. It is next to impossible to determine in advance how the current will be divided.

About your circuit, clearly, it contradicts the product sheet of the charger. The charger has to be parallel to the battery and the load, not in series.

The charger IC is programmed to a certain charging current, max 2A. This is only indirectly related to the supply current of the solar panels. If they can supply 4A, only 2A will be charged to the battey, the rest should be drawn by the load. If the load doesnt draw >2A at all times, problems may (will) arise.

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  • \$\begingroup\$ Ah, thank you! I didn't think about the voltage difference problem. You saved me a lot of trouble there. Regarding the circuit: Yeah, sorry, you're right. The circuit shown above was meant as a simple diagram of what I meant, not a real circuit (consider it as sort of a flowchart). Sorry about the confusion. \$\endgroup\$ – gspr Feb 26 '11 at 12:28

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