I have a solar "box" project with a 10 W, 12 V panel and I noticed something when I was testing the battery voltage (two 12 V battery in parallel). When I measure the battery voltage directly from both batteries, it shows 13.86 V, but on the charge controller battery input it shows 14.56 V. This only happens when the controller is fast charging, but when it isn't charging the battery (pulled out the panel cable) it's almost the same (0.1 V difference). When the controller is not connected to the battery at all, the voltage on the cable I used to connect it is the same voltage as the battery.

What I'm asking is, is this normal or is my controller acting up?

  • 1
    \$\begingroup\$ It is not normal to have such a large voltage difference. The problem seems to be that there is resistance in the connection between the battery and the controller. The voltage difference is euqal to the charge current * series resistance in the wiring. When the current is zero, the voltage difference is zero. \$\endgroup\$
    – user57037
    Jun 13, 2016 at 6:42
  • \$\begingroup\$ What @mkeith said so how thick and how long are OPs wires? \$\endgroup\$
    – winny
    Jun 13, 2016 at 7:57
  • \$\begingroup\$ i use a thin cable (10 fiber) , im not sure how thick but its as thin as cheap earphone cable, maybe thats the cause, ill look into it tomorrow \$\endgroup\$
    – Blur.png
    Jun 13, 2016 at 8:55

1 Answer 1


You need a diagram with labels to ensure that all your points are clear. As it is some words could mean several things.

Amps = Watts/Volts.
At full panel rated power (Wmp) the current is
~~ Watts/Volts = Wmp/Vp = 10 Watt/14.56V = 0.7 A.
You see a voltage drop of (14.56-13.86) = 0.7V .
Resistance that would cause this = V/I
= 0.7V / 0.7A = 1 Ohm.
It is "easy enough" to get 1 Ohm of resistance from cables and connections.
IF there is in fact a 0.7V drop across the wiring then thicker wires should help.

If the above incorrectly analyses the situation then you need to provide a diagram that better explains the situation.


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