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I want to control a 250W 24v motor, as well as a 12v 1W motor using a raspberry pi. It needs to all be powered by 2 lead acid batteries which are being charged when not in use. My plan is to use two of these.

One connected to each 12v lead acid battery, wired in parallel to the solar panel. For the 24v volt motor, I will wire both of the adapter things in series. For the 12v motor, I will wire it straight onto one of the the adapter outputs. For the raspberry pi, I will use a 12v-5v converter and wire it straight onto the other adapter's outputs.

  1. Will both batteries charge at the same speed?
  2. Will the raspberry pi or 12v motor get fried to a crisp?

Thanks.

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  • \$\begingroup\$ Why do you need 2 when the unit (claims) to work at 24V? Theoretically you only need one. My main concern would be that cheap controller. It is unlikely to be able to work at the currents you will need. I used something similar when I first set up my PV system and it didn´t last very long. Better to buy something more reliable. \$\endgroup\$ – F. Bloggs Sep 16 '17 at 12:26
  • \$\begingroup\$ Two because I need to use the 12v motor at the same time. I suppose I could use a potential divider, but I don't trust myself with one of them. The device has a rated current of 20A, I'll only be using 12. The reviews seem good, but if you don't think so could you recommend a better one? \$\endgroup\$ – JoeP Sep 16 '17 at 14:58
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One connected to each 12v lead acid battery, wired in parallel to the solar panel.

You can't do this, because these solar controllers are not isolated. Most of the controllers are not isolated for efficiency purposes.

For your power scheme where the maximum load is at 24V the most efficient method is to charge both batteries in series (24V) with 24V compatible solar controller and 24V compatible (like 28V MPP and 35-40V open circuit) solar panel. Then you get a DC/DC buck regulator with input from 20V to 30V and output 12V/1W+ (0.1A+) and another DC/DC buck regulator with input from 20V to 30V and output 5V/2A+ for the Raspberry.

The regulator for the 1W motor could be linear, but it would dissipate about 1-1.5W and for a battery operated equipment it is always better to look for the higher efficiency so a switch-mode DC/DC is preferred here. Also depending on motor's characteristics and especially start-up current you may need some power reserve.

For the Raspberry your only choice is to use switch-mode DC/DC because linears will have terrible performance.

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  • \$\begingroup\$ What do you mean by a 24v compatible solar panel? I thought they didn't have a specified pd. Can lead acid batteries not charge at any voltage anyway? \$\endgroup\$ – JoeP Sep 16 '17 at 15:38
  • \$\begingroup\$ The problem with solar controllers not being isolated is not quite clear to me. Do you mean that 2 controllers cannot be used without connecting their GNDs together? \$\endgroup\$ – Dmitry Grigoryev Sep 16 '17 at 15:41
  • \$\begingroup\$ @JoeP All panels are specified for maximum power and for the voltage at which this maximum power occurs - this is the maximum power point (MPP). It slightly varies with temperature but usually in +/-10% margin only. \$\endgroup\$ – Todor Simeonov Sep 16 '17 at 15:46
  • \$\begingroup\$ @DmitryGrigoryev using two (simple, cheap) controllers in one setup is tricky. They are not isolated. Most of them switch the negative path using N-channel transistors and also measure current through that negative path. Connecting PV- and B- of one controller to PV- and B- of another will lead to poor current measurement, poor regulation, etc. Connecting two controllers' outputs in series, while their inputs are in parallel (what was the OP's idea) is very dangerous - it will make battery short and maybe fire. \$\endgroup\$ – Todor Simeonov Sep 16 '17 at 15:55
  • \$\begingroup\$ So would a 12v pd going to the batteries not charge the them at all? \$\endgroup\$ – JoeP Sep 16 '17 at 16:12
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There's no guarantee for batteries to charge at the same rate: difference in loads an sunlight will quickly lead to dis-balance. It would be much better to keep the batteries balanced, since the 24V motor will draw current from both batteries and can damage the least-charged battery with reverse current.

It would be better to either wire the batteries in parallel and generate 24V using a DC-DC converter, or find a 24V charger which could charge both batteries in series, and generate 12V using a DC-DC converter.

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