NiMH is an abbreviation for Nickel-Metal Hydride. NiMH batteries are some of the most common rechargeable batteries we see in consumer electronics. Due to their superior chemistry, NiMH batteries have taken over Nickel Cadmium Battery applications. As they don't use Cadmium (a toxic chemical when used in battery applications) and additionally do not have the same memory issues that plagued NiCD, NiMH is clearly the better option of the two. Portable, high-drain power solutions are amongst the most sought after for battery applications, so we've put together this collection of tips for using NiMH batteries in your project! If you want to skip ahead and find some NiMH Batteries, we supply a full range of battery options from Pololu. You can see our range here.
What types of NiMH Batteries are available?
We typically see NiMH battery packs made up multiple individual cells connected in series (refer to diagram above). This is completely safe to do with NiMH batteries, unlike their LiPO battery counterparts. Each of these individual cells is rated for 1.2V, meaning we see NiMH battery packs rated at voltages that are multiples of 1.2V. In particular, we have 1.2, 2.4, 3.6, 4.8, 6.0, 7.2 and 8.4 Volt Battery packs available.
The idea behind this back of the envelope calculation is that the voltage of the battery itself comes from the chemical potential energy difference between the electrodes within. That means each NiMH battery cell will have that voltage rating of 1.2V no matter the physical size of the cell. What the physical size of the cell does indicate is the capacity of the battery. In general, the bigger the cell, the more mAh your battery will have.
A quick reference of this relationship can be seen in the table below:
What applications are suited for NiMH Batteries and why?
As we mentioned above, NiMH batteries are perfect for short-term (<30 days) high drain usage. Some consumer applications we see NiMH used in are digital cameras, communication equipment, personal cosmetic equipment and laptop batteries
Subsequently, what shouldn't you use NiMH Batteries for?
NiMH batteries do have a couple of flaws, mainly that they self-discharge. When the battery is not in use it will slowly deplete its charge and if it's left long enough your batteries can be permanently damaged. A rough estimation of the depletion in a NiMH battery is 20% of the battery level will deplete in the first 24 hours after charging, with a further 10% depleting per 30 days thereafter.
How do you recharge NiMH Batteries?
To charge a NiMH battery you'll need a specific charger as using the incorrect charging method for you battery can render your battery useless. Our top pick for NiMH battery charging is the iMax B6 Battery Charger. It supports charging batteries up to 15 Cell NiMH Batteries and has a bunch of settings and configurations for different battery types too. Be sure to only charge your NiMH batteries for a period of fewer than 20 hours as prolonged charging can harm your battery!
How many times can you recharge NiMH Batteries?
Typically, we expect 2000 charge/discharge cycles out of a standard NiMH battery, although, your mileage may vary. This is due to the fact that each battery is not identical. The usage of the battery may also dictate the number of cycles the battery will survive. All in all, 2000 (or thereabouts) cycles from a battery is quite substantial for a rechargeable cell!
Charging Considerations for NiMH Batteries?
There are a couple of considerations that you should bear in mind to protect your battery's lifespan:
- Trickle Charging is the safest method you can charge your battery. To do so, ensure you are charging at the lowest possible rate that will keep your overall charge time BELOW 20 hours and remove your battery at that point. This method will essentially be charging your battery at a rate that will not overcharge your battery but keep it topped up.
- Do not overcharge NiMH batteries. Put simply, this means that once the battery is fully charged you stop charging it. There are a few ways to know when your battery will be fully charged, but it's best to let your battery charger handle it. Newer battery chargers are "Smart" and can detect small changes in the Voltage/Temperature of the battery that will indicate a fully charged cell.
Memory of NiMH Batteries?
Originally, there were widespread problems with Nickel based batteries and memory. Essentially, if you didn't fully deplete your battery before charging it, you would lose a part of the capacity of your battery. Over time, this would render your battery a big chemical filled paper weight. The NiMH batteries we see today do not have these problems, although if you aren't fully discharging your battery every use you can still see the same effect taking place. Newer NiMHs can be restored by "exercising" the battery (fully charging and discharging the battery a few times).
Substituting Alkaline Batteries with NiMH Batteries?
This is perfectly okay to do! If you are burning through a ton of AA batteries, you can pick up some NiMH batteries to substitute in place of them. The difference in voltage (Alkaline 1.5v, NiMH 1.2V) is negated by the voltage drop that alkaline batteries experience through use.
That has just about covered everything you need to know, at a glance, to do with NiMH batteries. Like we said earlier, every battery is a little bit different and the quality of the batteries are usually a function of the manufacturer of the battery. Be sure to check out your batteries datasheet/product information before charging it up for the first time, the last thing you need to do is brick your brand new battery pack! Thank you for taking the time to learn about NiMH batteries, let us know if this article was helpful to you or if there's anything else we should include!