After writing a post about my experience with LiPo Batteries I’m now proposing a post about NiMh. LiPos are good batteries : not expensive with a good energy density, really easy to manage for charging. For these reason they are used in many electronic designs.
But in some applications they are not enough stable, this is as an example the case for an automotive use where the internal temperature of a car can reach higher temperature than 65°C. This is the limit for LiPos. In my experiences I had some having been destroyed due to warm temperature.
NiMh is not an optimal solution but you can find some batteries capable to support 85°C and resist to +130°C before being destroyed and have a fire risk. Varta V500HT is a good example of battery for a such use.
NiMh is less easy to use than LiPos, this post is detailing my experience.
Different kind of NiMh batteries
For using in usual circuit we need a power source upper than 3,2V. Compared to LiPo providing 3,7V, NiMh solution requires to use more than a cell. Every cell is providing 1,2V. Basically we need to use 3 cells to provide a sufficient voltage or to use a step-up dc-dc converter. It makes NiMh powering solution expensive.
The first NiMh I’ve tried are noname one found on aliexpress for a price around 5€. On 4 received, 3 were working. These battery are given for 400mAh but my test shown you should more expect about 250mAh of capacity. The size is interesting with half AAA form factor you have a small and compact solution.
The second type of batteries I’ve test are based on VARTA V500HT. The price is really higher with about 15-20€ per pack to get 500mAh. The good news is that they really provides the expected 500mAh. Form factor is also larger than the no-name one.
Compared to LiPo the choose of NiMh have a large impact on final product price.
Charge NiMh batteries
The problem of charging NiMh is the battery voltage is not in relation with charge progress.
In theory the charging curve (from this excellent website) looks like the above graph. When for a LiPo you just have to check the voltage to know when the battery is charged, here you can’t as the voltage decrease after reaching the maximum. So there are different way to manage that :
- Check dv/dt and detects when it starts to be a negative value
- Check battery temperature as once the battery is charged the temperature is growing
- Charging time when you have a good control of your battery status.
- Charge at C/20 to avoid temperature growth and limit need to manage anything…
These method are usually implemented in the NiMh charge controllers.
Charge NiMh batteries in a complex environment
All these methods works well if you are making full charge in a stable environment. For my personal use it was more complex : I’m not managing the charging source and it can be up or down whatever in the middle of the charge. More over I’m expecting to not discharge the battery to it’s minimum level but I’m looking for keeping it as charge as possible… Temperature is also out of control as the environment can be warm or cold at anytime and change quickly.
The solution was for me to charge the battery up-to the level under the pic of voltage for ensuring to never be in the end of charge condition where the battery over heating. The question was : is the used capacity higher enough at a such voltage.
This curve is a clean curve obtained from a Varta 3xV500HT @80mAh current. The curve is like the theorical one. My assumption is a Voltage of 1750(units) could be a good stopping point for charging. This level is corresponding to 4,4V and 360mAh of capacity on the 500mAh (72%)
With noname battery the level sound likes different and it can vary from a battery to another : as you can see on the graph above the 1750 level may have given different results : on blue line it can stop charging the battery at 50mAh (25% of real capacity) when on red line it correspond to 191mAh (80% of real capacity)
As seen this method is not working correctly every time and should evolve with battery age or usage conditions. Basically it can works but not easily.
As a conclusion, charging NiMh in complex environment is a complex question with complex solutions even if you accept to not have a full use of the battery capacity.