I was going over a request for a solar battery charger, and one of the points was a Low Voltage alarm (possibly meaning a LVD) to be triggered at -48VDC. This is a bit confusing as, to the best of my knowledge, most battery chargers and banks are rated at positive voltage such as +48VDC.

I know that +/- is mostly a matter of where you consider the ground to be, so does this mean that their battery bank is grounded at the positive pole? Or what is the usual way of connecting a battery bank of 48V to equipment that is working with -48V?

I am trying to figure out why they require a negative voltage for the LVD, basically.

  • \$\begingroup\$ If the total combined battery voltage is 300 V (!), LVD might be required to be activated around 252 V.. Do you have any spec on the battery which has to be charged? \$\endgroup\$ – Umar Jun 26 '15 at 7:20
  • \$\begingroup\$ The batteries to be charged will probably be a bank of 2V2000Ah batteries, meaning a series of 24 cells to provide the full 48V2000Ah. The LVD is being used to make sure the batteries won't go below a certain depth of discharge (as the voltage drops slightly as the batteries discharge), so I am guessing that the 48V is going to mean somewhere between 50 and 70% DoD. Problem is that whenever I faced this issue before it was never required for it to be negative 48V, so this confused me. \$\endgroup\$ – Teh Cube Jun 26 '15 at 7:27
  • \$\begingroup\$ Ask for clarification from your client. It might be just an hypen and not a negative sign \$\endgroup\$ – Umar Jun 26 '15 at 8:18
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    \$\begingroup\$ It's common for telephone equipment to run at -50V (which, in practice would be -48V when running on batteries). It does simply mean that the positive terminal is the one that's grounded. \$\endgroup\$ – Simon B Jun 26 '15 at 8:34
  • \$\begingroup\$ Thank you both, I will try and clarify with client then. \$\endgroup\$ – Teh Cube Jun 26 '15 at 8:44

Voltage is relative - it represents the work done by a charge to move from one point to another. If you're standing on the negative terminal of the battery, and you call the part you are standing on "ground", or 0V, the positive terminal will look positive relative to you. Standing on the positive terminal, you might call that terminal 0V, and then the other terminal would have a negative voltage.

Cutting a long story short, you can go ahead and design your battery charger as usual - with positive voltages - and just swap the wires going to the device consuming the power.

This works because what you consider to be the positive rail, the supply circuit will consider to be ground, and then what you consider to be ground will be negative with respect to that.

  • \$\begingroup\$ Thanks for your reply! I think I understand what you are saying. The issue is that I will not have access to client's equipment, I am only supposed to provide the charger based on their specifications - I am just a middle-man in this scenario, that's why I lack in-depth knowledge on the subject. What I am worried about is that they don't damage any equipment by either hooking up the battery bank to the equipment in the wrong way, or hooking up the charger to a not-fully-empty battery bank incorrectly. Thus I will try better understand their system first. Thanks again! \$\endgroup\$ – Teh Cube Jun 27 '15 at 14:57

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