Nimh Battery Packs

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Rebecca Donnelly

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Aug 4, 2024, 1:40:58 PM8/4/24

to urmannehou

Afterseveral of searches on the internet for information about charging NIMH batteriers, i feel the need for some clarification. I've gotten my hands on a bunch of battery packs. Each battery pack is 24V 1500mah, they are built on 20 x 1.2V 1500mah GP AA batteries.

Yes. However, make sure they have the same voltage before you connect them in parallel. Either charge/discharge to the same voltage, or connect them with a resistor to limit the current while they equalise, before connecting properly.

Given the high current capability of NiMH, it would be very wise to include a fuse in series with each battery. If one pack suffered a short in some of the cells, the other 9 could gang up on it to drive a fire-starting current through it.

While individual NiMH cells don't get damaged even if abused down to 0v, a battery of series connected cells can reverse any single cell that discharges first, so sending it below 0v. This can happen even with an apparently adequate voltage on the battery pack. Unless you monitor the voltage of individual cells, you will need to stop discharging at a very conservative threshhold, at least 1 V per cell, to reduce the possibility of this sort of failure.

The other way to reduce this sort of failure is to balance the cells with a proper overcharge, 16 hours at 0.1C. This is why we only hear about cell balancing with LiPos, as it happens easily and automatically with lead and nickel chemistries with a simple overcharge, not possible with LiPo!

Neil provides a complete answer.Be sure to take his cautions seriously, in certain circumstances batteries are dangerous.The cutoff sensor/circuit at low voltages is very important. You really want to avoid "over discharge". In the old days of hermetically sealed NiCds in spacecraft, driving a cell negative, as Neil points out from over discharge, generates oxygen, hydrogen and heat. In other words, it will explode. BTW, you don't have that problem because terrestrial cells are never really "sealed". They have a pressure relief valve.

What I will add, is that you need to look at the full I V characteristic of the solar array (SA) that you are thinking of using. 24 vdc might be voltage where max power is derived from the SA, but in a lightly loaded case the voltage will be quite a bit higher so ensure that your charge regulator can handle that voltage. If at all possible, i.e. if you design your own battery charger, include temperature sensing of the cells, and adjust your final voltage according to the spec for NiMh chemistry.

Custom Nickel Metal Hydride (NiMH) battery packs are an increasingly popular choice for many applications due to their low cost, durability, and low operating risks for the end user. This chemistry type can offer a wider operating temperature range than some lithium batteries, which, combined with the much lower development and product costs, makes NiMH the product of choice for many commercial and industrial applications.

NiMH battery packs prefer a constant current charge where a standard charge can be C/10 for 14 hours. NiMH batteries can take a charge in as little as one hour with dT/dt (delta temperature/delta time) or peak voltage detect termination, along with several other metrics that can be monitors. It is always a good practice to incorporate a timer as backup in case the other methods fail to activate.

Typically, the protection and BMS circuits are more commonly used with lithium chemistries in order to mitigate the risk of fire in custom battery packs. To help increase the safety of NiMH batteries and extend service life stand-alone and integrated PTC (positive temperature coefficient) protection devices as well as PCM (protection circuit modules) are available in custom Nickel Metal Hydride (NiMH) battery packs.

Our engineering team is experienced in developing NiMH PCM and BMS systems to fit virtually any application in a very cost-effective manner. PTC provides temporary over-current protection with auto recovery where a BMS typically handles the same protections found with PCMs using a microprocessor. This allows for additional monitoring, control, and reporting of a wide variety of parameters and allows users to gain real-time access to an abundance of data through traditional communication protocols (I2C/CAN bus).

As portable electronic devices have become smaller, lighter, and more sophisticated, the batteries that power these devices have had to provide ever-increasing levels of energy. Our battery design team uses the latest design tools to optimize the reliability, safety, and manufacturability of your custom NiMH battery packs. While NiMH is not as closely regulated as lithium products, there are certain transportation regulations that need to be followed when shipping Nickel Metal Hydride (NiMH) battery packs.

Our assortment of rechargeable NiMH batteries and battery packs gives you variety of options for powering your robot or electronics project. The battery packs are composed of 1.2 V cells and are available in several different capacities. These packs are well-suited for powering small robots, and the smaller (1/3- or 2/3-AAA) packs make great power sources for miniature robots, such as those that might be based on our Baby Orangutan robot controller.

This is a generic rechargeable AA NiMH battery with a capacity of 2200 mAh. These batteries are well-suited for powering small- to medium-sized robots and can be used with our AA battery holders. You will need to charge these batteries before you first use them.

This is a generic rechargeable AAA NiMH battery with a capacity of 900 mAh. These batteries are well-suited for powering small- to medium-sized robots and can be used with our AAA battery holders. You will need to charge these batteries before you first use them.

So, after spending a king's ransom on AA batteries from Costco for the past couple years, I decided to buy four Garmin NiMH battery packs for our two Oregon 700 receivers. They arrived today, and I have two questions:

When plugged in to a USB power source, is there any indication on-screen that charging is actually happening? On virtually every device that I own, from game controllers to my cell phone, there is an onscreen indicator that the battery is charging. I could see nothing of the kind on my Oregon, and the information on the Garmin website says nothing about this one way or the other that I've been able to discern. I think I figured this part out - I was plugged into a USB multi-port brick. When I plugged my Oregon into my computer, I got the request asking if I wanted to open USB storage. I tapped "no," and now there's a charging icon where the battery indicator is.

On one of our two receivers, when I install the NiMH battery pack, the option under "Settings" for "Battery Type" disappears, as in it is not even a listed option. Upon removing the battery pack and replacing it with with regular alkaline AA batteries, the option returns. This is puzzling, to say the least, because the option is clearly available on my other Oregon 700. Any thoughts?

An additional, related question: on the receiver that is having the battery type display issue (which we'll call unit "A" for clarity), I end up with this situation:

When unit A is plugged into my computer, it immediately goes to USB mass storage mode. Unit B (the one that lacks the battery type display issue) always gives me the option of either going into USB mass storage mode or not (choosing "no" is how I got the charging icon in the main battery indicator). Both units have are completely updated (software version 5.30). What the heck?

Why do you expect the GPSr to show you AA battery type configuration options in the Setup Menu when the Garmin Lithium-ion Battery Pack is installed? The Garmin battery pack is designed to depress the little rubber nub in the center of the battery tray so the GPSr automatically knows NiMH batteries are installed. If one of your GPSr is still showing the battery configuration option, something is broken. Check out the Operation > Power Sources page here. All of these question will be answered, and more.

This behavior is based on the Interface option you have selected in Setup > System. Garmin Spanner always asks you what you want to happen, the others all go directly into Mass Storage Mode. Please see the link above for additional related information.

That's a pretty cool feature -- install the official battery pack, and it mechanically presses the switch to tell the device that the pack is there. The switch is the little rubber bump in the center of the battery compartment.

Well, my rationale for asking the question is that the situation is different on two seemingly identical units, but the info provided by kunarion above gives me a lead. My unit is set to Garmin Spanner, while hers is Garmin Serial.

I just made the switch on hers, and it now asks for what to do when USB is plugged in. And, in a bonus, it shows a *different* battery charging icon - it shows a standard "battery charging" lightning bolt. Perhaps it has something to do with the fact I'm using the Classic mode and hers is set for Activities.