Recharging NiMH Batteries
Fast charge rates are usually best for NiMH batteries. However, a rapid charge rate causes the battery temperature to increase. As long as the temperature of the battery does not exceed 140°F, the battery is usually safe. Temperatures above that may rupture the battery. Charging at rates above 1C (one times its capacity rate) can exceed the rated battery temperature causing rupture. This is due to excessive current being driven into the battery. This can be done with both a wall charger and a car charger. However, most NiMH battery manufacturers advise against using a car charger to recharge their batteries. Let’s explore why.
Let’s assume you’ve used your e-caller all day and the batteries are depleted. Let’s say that the batteries have discharged from 1.25V per battery to 1.0V per battery, and your e-caller uses 8 AA batteries in its battery pack. That makes the total depleted voltage charge 8.0V (8 batteries at 1.0V per battery). This is a realistic situation but keep in mind that each battery has its own charge specifications which may not be exactly 1.25V. Your car battery runs anywhere from 12.8 to 14.5V. For this example, we will assume that your car is only running at 12.8V. Each NiMH battery in your e-caller has an internal resistance of 0.40 Ohms. The total resistance for the 8 cell pack is then 0.32 Ohms. Using Ohm’s law (the most basic and essential equation of electricity) which states that the current is equal to the voltage change divided by the resistance, we can determine the current that will run through the battery pack. Doing the math, you get a whopping 15 Amps! Normally, this current is short-lived as the induced surface charge quickly brings up the cell voltage which then reduces the current before everything can spiral out of control. There are other factors that are also at work here, but this is the layman’s version of the process. So far, so good.
The real problem is if there is a single bad battery in the pack. If one of the batteries in the pack of eight has been deeply depleted or has gone bad, it won’t accept the surface charge and the current won’t decrease. This increases the battery temperature until the battery ruptures.
To counter this problem, e-caller manufacturers add a thermistor to the battery pack. A thermistor is a resistor that increases its resistance with increasing temperature. However, this is a band-aid approach to solving the problem. If the thermistor fails, there is nothing to stop the batteries from overheating. Moreover, a thermistor, without the addition of some extensive circuitry, is really an afterthought. When the batteries are already too hot, the thermistor kicks in and stops the voltage. Unfortunately, it is often too late and the batteries could still start a fire. Charging battery packs have been responsible for a few car fires, and I have heard of a few e-callers melting because of this effect. Burnt plastic smell is hard to get out of a car.
Even if the batteries do not rupture, you still run the risk of ruining the entire battery pack. If one of the cells in the pack is bad, then the entire battery pack will fail to reach the voltage required for shut off causing current to continue being driven into all the batteries until the entire pack is ruined.
To avoid the problem, each battery should be charged individually by a recharging unit that analyzes each cell to determine its state of health using both change in temperature over change in time and change in voltage over change in time algorithms.
Most NiMH battery manufacturers recommend a maximum charge of 1.2 Amps for 2-3 hours of total charge. Do not over-charge NiMH batteries using a poorly designed wall or car charger.
Be Careful of Recharging Your NiMH Batteries in a Battery Pack Using a Car Charger
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Category: Dr Shawley's Blog
I use a lot of different battery technologies as part of other hobbies I have. So whilst I agree with the article, let me add a few cents.
NiMh batteries provide much higher storage capacities than traditional NiCad. They also tend to hold voltage longer (under load) than NiCad. To simplify, a 800mAh NiCad will see the voltage fade as it discharges whereas a similar NiMh will hold full voltage longer…then fade rapidly at the end.
Another thing to consider is the batteries designed current draw. One AA may support high discharge rates where another won’t. Note that most ‘domestic’ grade batteries don’t support high discharge rates. Many hobby batteries are designed for higher discharge rates.
Fast charging is possible with most NiMh, but to gain maximum cycles/life try lower charges. Personally, I use no more than 800mAh charge rates for all NiMh AA sizes.
NiMh batteries also heat whilst charging, and you may even notice they drop back to ambient as they approach full charge. As long as they don’t get too toasty they should be fine. Warm is better than hot.
Another warning for NiMh is to be aware that some discharge whilst stored. Typically, higher discharge rate batteries will also discharge faster whilst stored. If your batteries fade quickly when stored they are at the end of their life.
The best way to charge batteries (if you depend on them) is to buy a specialised charger. Many R/C hobby stores sell models that will cater for everything from NiCad, NiMh, LiOn, LiPo, LiFe, and A123. These often run off 12VDC which makes hooking one up to your car (or car battery) a breeze. You can buy decent quality computerised chargers for as little as $50-$75. Some will even provide individual cell balancing.
In summary, if you depend on batteries for hunting/camping etc then it may be worth buying speciality batteries. They are generally better quality and capacity (see DSE, Jaycar or similar). Buy a good hobby grade charger, they are much more capable and relatively inexpensive. Finally, keep the charge rates sensible (I like 800mAh for NiMh).