How do you protect the battery?

The goal of a charge circuit is to protect both the battery and your device. If you don’t have charge protection, you can damage the cells on a battery or cause a fire. Please use caution whenever charging batteries from solar power (or any other power source), they are not toys.

The activities mentioned here require special test equipment to show how the batteries behave under different conditions. If you don’t have the equipment, you’re more than welcome to follow along or perform simplified tests with a multimeter.

Failure to use a charge control circuit will adversely affect the performance of your battery over time in the best case scenario. In the worst case scenario, cells may explode, catch fire, or otherwise cause serious harm and damage to people and property. By avoiding these hazards, usable battery life can be preserved and extended. The best battery chargers will implement a range of protections and detection methods to rapidly and safely charge your batteries without damaging them.

Typical Protection offered by charge controllers and power management electronics:

Over Charge

Voltage and/or Current regulation is used to prevent the cells from taking on too much power, too rapidly, and overcharging. In simple circuits, linear devices such, as regulators, are used to limit voltage and/or current. In more advanced circuits, a microcontroller is used to monitor and control charge functions. The protection method varies according to cell chemistry: NiMH batteries require a circuit that detects a change in voltage or temperature, where Li-ion cells require a two-stage constant-current/constant-voltage charge method.

Over Discharge

Discharging the battery will cause the cell voltage to drop. The over-discharge protection monitors the cell voltage, turning off the output when the voltage drops below a preset “off” threshold. The output will typically turn back on once the cell voltage rises above a preset “on” threshold. Over Discharge protection can occur prematurely (before the cells are completely discharged) if the power consumed by the load is too high, but not high enough to trigger the short-circuit protection.

Short Circuit Protection

When a load draws too much current from the output of the battery, the output is turned off. Sometimes this feature will require a manual reset of the battery, including disconnecting and reconnecting the load.

Over Temperature

If cells are charged or discharged rapidly, the chemical reactions taking place inside may generate excessive heat. Over temperature protection in the form of either a thermal circuit breaker or a microcontroller reading the changes of a thermal resistor will disable the input and output of the battery until the temperature has returned to normal operating conditions. In the case of NiMH cells, a sharp increase in temperature is usually associated with over charging, in which case the charge rate should be drastically reduced or terminated.

Here are images of Li-ion cell protection employed in our V15 USB Battery. On the left is a circuit board containing electronics that protect the cells from overcharging and over discharging. On the right is a thermal breaker. If the cell temp exceeds 80 degrees Celsius, the break opens the circuit, disconnecting the cells from the rest of the power management circuit. When the breaker cools, it automatically resets.

WARNING – The following activities are demonstrations of circuit protection implemented in Voltaic Systems batteries. It is not recommended to try these with other batteries as you may permanently damage them.

Activity 1 – Over Discharge and Short Circuit Protection of the V72

    What you’ll need:

  • A programmable electronic load (one that will allow you to adjust the resistance OR the current drawn from the battery).
  • A V72, V44, or V15 battery.
  • Some way of connecting the output of the battery to the programmable load.

Watch what happens when resistance in the load is dropped simulating a short circuit.

Activity 2 – Testing Overcharge Protection

    What you’ll need:

  • A programmable power supply (one that will allow you to adjust the voltage and limit the current).
  • A V72, V44, or V15 battery.
  • Some way of connecting the output of the battery to the programmable load.

Watch what happens to current flow into battery as the battery becomes full. (Note: Li-Ion batteries typically measure Voltage of the cells to determine when they are reaching full capacity. Other batteries such as NiMh will use different methods such as Delta-V.)

This post was updated from the original post in March 2012.

10 Responses


    Nice- fills in the info that should be supplied with each system in brochure instruction booklet but isn’t supplied and why not?

    • admin

      Hi Mark,

      We try to balance between creating something that has every detail, but maybe too much for some people. We try to include everything we can think of on our site. /jeff

  2. Scott

    So the Voltaic cells have built in diodes so we can put them in parallel without problems that usually occur (reversal of shaded cell killing the output of all the rest)?

    • admin

      Hi Scott,

      The circuit boxes currently have diodes in them, but not the panels.


  3. john

    I have a unique problem that you will not ever here of again. I have talk to 3 different companies and got three different answers. I would like to build a solar panel to charge my solar lights mounted around the outside walls. I have 6 total to be charged on one panel. Each of the lights have 5-led’s that run on a 3.2v 300mAh lithium po4 battery. One guy tells me to buy a 18v10w panel and wire the lights in series. Another guy says buy 2- 18v panels with a controller that he did not have. And the last guy told me to build a panel with 45- 3×6 cells with a controller of some type. I am pretty sure one of these guys is going to burn my house down. Without go to college for the next ten years, is there a simple way for me figure this out. What size panel, what controller, in series or parallel

    • admin

      Only 300mAh? That is a small battery and you would want to use a charge controller so you don’t overcharge the battery. We don’t have one for LiP04 batteries either.

  4. Grant

    Hi Guys, I’ve just found your blog while researching PV battery charging using small panels and batteries and want to say thanks for taking the time to set out so clearly your expertise in this area in the 4 part tutorial. Clearly you didn’t have to do it but the fact that you have shows to me you care about your customers more than just what they buy. I know which brand I will choose when I next need to buy portable field power solutions and great to see you have a supplier already in New Zealand. Brilliant!!!

  5. Chris


    I have bought 3 of your V15 batteries so far. I have one question regarding the “Always on Mode” feature

    1) When in always on mode, what will happen to the battery when an overdischarge condition occurs? Will the load be turned off ?

    2)If so, then when the internal battery voltage has increased due to a subsequent charging, will the unit turn on the load again ?

    3) If the load is turned on again, then will the unit enter always on mode again ? I want it to stay in always on mode, but I am not sure how your firmware is written.

    4) I have noticed that under some peculiar sunlight illumination conditions (when in always on mode and when the solar panel is receiving spurious solar illumation (fast moving shadows for example, or overcast cloudy day), the unit will sometimes die on me and not respond at all with the load disconnected (the unit was fully charged_. Pushing the power button does not light up the LEDs whatsoever. The only way to “repair” the unit is for me to force a hard reset on your MCU by opening the cover, disconnecting the daughter board from the mainboard, and replugging the daughter board onto the mainboard again. Do you have any fix for that ? Have you observed such situation ?

    The daughter board has “RF967-B REV B0 2014.10.31” written on it. The mainboard has “RF967-A REV A3 2014.06.16” written on it.

    • Voltaic Systems

      Hi Chris,

      I”m going to send you an email, with these same questions and we may be able to respond a bit faster.

      1. When the V15 is nearly empty, it will shut off and preserve some battery power. Once empty, it will last about 30 days with no power and still be able to restart into Always On mode
      2. Yes, the V15 is designed to turn on once a charging buffer has been built up
      3. It should and again, our tests show it returning to Always On mode after 30 days of no charge – if you are seeing different behavior, please let us know
      4. We haven’t seen this behavior – but will try to replicate it. Are you seeing this on all V15s or just one of them?



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