Nimh battery charging amps. Q: Do NiMH batteries lose capacity over time?

Battery Charging FAQ

    Dumb Chargers (aka Manual Chargers). These chargers never stop. The user manual will estimate how long you should leave various size batteries in for, and you’re supposed to take them out exactly after that much time has passed. Since every battery is different, then no matter how diligent you are, you’ll either undercharge or overcharge your batteries this way. Undercharging means you get less run time, and overcharging means you reduce the batteries’ cycle life. Dumb chargers are also slow, because pumping the juice in fast when there’s no automatic shutoff could overheat the battery, cause it to pop its seals and start a fire.

If your NiMH charger is Smart, then you can mix NiMh and NiCD, different sizes, and semi-fresh and nearly-dead batteries at the same time. If it doesn’t then you can’t; if you insist on mixing them anyway, some of your batteries will be undercharged and some will be overcharged. more so than usual.

Is it better to charge fast or slow?

First of all, you almost never have a choice, so for practical purposes this rarely matters. Most chargers don’t let you set the charge speed.

But let’s say you have a fancy charger that does let you set the charge rate. Even then, the manufacturer default is likely what’s best for your battery.

  • For AAA, go with 200 mA. 500 mA is acceptable but could reduce cycle life. Never use 700 mA.
  • For AA go with 500 mA. 200 mA is so slow that it could make the charger miss the cutoff signal and overcharge the battery. (NLee the Engineer) 700 mA is acceptable.

At the recommended levels, a battery will take about 4-5 hours to charge. Faster than that potentially reduces cycle life, and too much slower than that could possibly cause the charger to miss the cutoff, overcharging the battery, and reducing cycle life.

Reviving batteries that won’t charge

    Dumb Charger method. If you have another, cheaper charger that doesn’t test the battery voltage, just stick in the dead battery for a while until it’s charged enough to be recognized in your good charger.

Jump-start methods. To give you battery a jump-start, you connect the terminals of a fresh battery to those of the dead battery (positive to positive, negative to negative). You keep them connected for a few seconds, or for just a fraction of a second if you’re using a power tool battery as the good battery. After jump-starting, try to charge the battery again. If the charger still doesn’t recognize the battery, then try to jump-start the battery again, this time for a longer length of time (or more swipes in the case of the power tool battery). While I’ve never had any problem using these methods, they’re definitely use at your own risk. Anyway, here are various ways to do a jump-start:

  • Any metal method. Stand the two batteries up on a metal surface like a stainless steel countertop or cooking pan(not non-stick). If you can’t find a metal surface then lay a paperclip down on the table and then presto, you have a metal surface. Then lay something like a screwdriver or knife across the positive terminals. Alternatively, you could lay the batteries on their sides and touch one screwdriver or knife across the positive terminals, and a screwdriver or paper clip across the negative terminals.
  • Paper Clip method. UNPLUG your charger! Then insert a freshly-charged battery and your dead battery, side by side. Unbend a paper clip and touch the ends to the positive end of each battery for a few seconds. If you notice the paper clip getting hot, then the dead battery has a short and you should stop trying to charge it and just recycle it. Thanks to NLee for this idea.
  • Battery Holder method. Get a 2-cell battery holder. Insert a freshly-charged battery and the dead battery in the same direction. Since you’re inserting one of the batteries opposite of the normal way, make sure the spring is contacting the positive head. Touch the black and red wires together for a few seconds.
  • Power Tool Battery method. This method involves sparking and seems dangerous, so this is definitely use at your own risk. Anyway, remove the battery from a power tool such as a cordless drill. Put the dead battery in a battery holder, hold the red wire to the positive terminal of the power tool battery, and then briefly touch the black wire to the negative terminal of the power tool battery (less than one second). When you brush the negative wire to the power tool battery you’ll likely see a spark. One swipe is likely enough, but I often do 2-3 for the heck of it.

Charging Methods

Not every battery has the same charge rate or charge time. So, they require different charging methods to ensure the battery isn’t undercharged or overcharged or that there is a reduction in the number of charge cycles. There are different charging methods that can be applied to certain battery chemistries.

Trickle Charge

A trickle charge is where the charger provides a very low voltage that is usually equal to the self-discharge rate of the battery. It is normally set at a rate of 0.05C and can go up to 0.01C. Trickle chargers are used to prevent overcharging when batteries are placed into storage. At times, a trickle charge can cause batteries to become too hot. In addition, the slow constant charge can cause memory effect problems for certain battery chemistries. A trickle charger can take 14 hours or more to fully charge a battery.

Rapid and Fast Charges

Rapid charges and fast charges offer higher voltages over a shorter amount of time at a constant rate. The Rapid charger has a rate of 0.3C to 0.5C; a fast charger has a rate of 1C. Overheating can occur to some battery chemistries, which could result in damage, so the battery must have a cooling down period before the charge is applied again. Other batteries respond well to Rapid and fast charges. Another issue with fast charges is that battery chemistries can become unstable, and this may lead to a rupture, leak or explosion if the battery is left on the charger for too long.

Step-Differential Charges

A step-differential charge is a method where a fast charge of 1C is applied to the battery. When the battery reaches a specific charge threshold, it is allowed to cool off. It is placed into a rest phase as a lower amount of charge is added. Then, as the battery reaches its next charge threshold, the charge is lowered more until the battery chemistry reaches a full charge.

Ultra-Fast Charges

Ultra-fast charges are newer charging techniques where the charge voltage will be set at 1C to 10C. These types of charges are normally reserved to specialty batteries as the batteries can be at a 70% charge from 10 minutes to about 60 minutes.

Charge Rates and Charging Times

When comparing NiMH to lithium chemistries, both batteries respond well to Rapid charges and fast charges without experiencing a significant loss in capacity or a shortened service life. Knowing the different charge rates and times will allow you to get the greatest capacity in the right amount of time.

NiMH Chemistry

This chemistry can go through Rapid charge, fast charge, ultra-fast charge or step-differential charge methods. Charge rates and times are:

  • Rapid Charge: A charger can provide a rate of 0.3-0.5C as it will take 3-6 hours to charge.
  • Fast Charge: Charger can be set at 1C charge rate as it can take from 1 hour or more to fully charge.
  • Ultra-fast Charge: A charger can have a charge rate of 1C up to 10C. The battery can be charged up to 10 minutes to an hour as the state of charge (SoC) will only reach 70%.

Lithium Chemistry

Lithium chemistries can experience a Rapid, fast and ultra-fast charges. A slow or trickle charge should be avoided as too low of a voltage will cause degradation and instability.

  • Rapid Charge: A charger can have a set rate of 0.5C as it will take 3 hours or more to charge.
  • Fast Charge: Lithium chemistries can take charge rates of up to 1C, although the maximum for safe charging can be around 0.7 to 0.8 depending on the battery pack. The charging time can range around 1 hour.
  • Ultra-Fast Charge: There are ultra-fast chargers available for lithium battery chemistries. Like NiMH, the charge rate can go from 1C to 10C, although a lower charge is advisable. It can take an hour or less to charge the battery. This method is reserved for specialty batteries.

Understanding Battery Management Systems

Battery management systems provide a range of benefits to battery packs depending on the brand. Each BMS can have different features, yet all will provide you with the SoC of the battery as well as protection from overcharging and overheating to prevent thermal runaway. Additional features for some BMS technologies include state of health (SoH) diagnostics, cell balancing, cell protection, and charge control.

While the battery charger will have a sensor to measure temperature and the applied current to adjust the voltage, a BMS control provides additional benefits to increase the safety and protection of the batteries. Lithium chemistries require a BMS to monitor the charge voltage, so it doesn’t become too low or too high.

Q: Can I use a higher rated mAh battery in my electronic device (i.e. 1800mAh vs. 2000mAh)?

A: Yes, the mAh rating will give you longer run times between recharges. The higher rated mAh of a battery has no effect on electronic devices other than they allow longer term use.

A: In fact, over the course of their discharge, alkaline batteries actually average about 1.2 volts. The main difference is that an alkaline battery starts at 1.5 volts and gradually drops to less than 1.0 volts. NiMH batteries stay at about 1.2 volts for almost 80% of their discharge cycle. Once alkaline batteries discharge to 50% capacity, it will be delivering a lower voltage than a NiMH battery.

Q: What you NEVER want to do with replaceable batteries?

  • Never mix batteries from different manufacturers
  • Never mix batteries of different capacities
  • Never mix batteries of different chemistries, i.e. NiCd, NiMH, Lithium, etc.
  • Never DROP the battery if you can help it as NiMH batteries damage internally quite easily
  • Never store NiMH in the refrigerator
  • Never expose to extreme heat

A: Yes, but nothing drastic. About 10 to 15% of the battery mAh capacity will be lost at the 400 to 800 recharge level. This will vary greatly because of battery and charger quality, along with how the consumer treats their batteries.

Q: When I receive my batteries do I need to charge them?

A: Yes, before you use them for the first time, you need to charge your NiMH batteries fully. Please note that for new NiMH batteries, it is often necessary to cycle them at least three to five times or more before they reach peak performance and capacity. The first several times that you use your NiMH batteries you may find that they run down (discharge) quickly during use. Don’t worry, this is normal until the batteries actually structure internally.

A: Yes, there are differences in the different chargers on the market today. If the charger was designed and sold in the past couple years and specifically says it is made to charge NiMH batteries you are probably okay. Most of the new chargers use a small computer chip to manage the charge and you should be getting at least 500 charges from your batteries. If not, buy a new charger. Some of the no name batteries sometimes have a short life. Fast chargers also tend to give shorter battery life of less than 500 charges.

How to avoid voltage depression?

The first two things you’ll probably read when you start browsing online are terms such as “voltage depression” and “memory effect”. Depending on what you’re reading, those two can sound like devastating issues that plague every battery owner and lead to countless battery failures.

That’s not exactly, the case, however. For starters, NiMH batteries don’t have a “memory effect” – NiCd batteries do, not NiMHs. As for the voltage depression issue, that can happen to RC NiMH batteries but it’s not all that common and it’s easily avoidable. Here’s what you need to know about it.

How does it happen?

When a rechargeable battery is repeatedly overcharged, the two metal electrodes can start getting covered by small rigid crystals from the electrolyte liquid. That’s not dangerous to the battery’s health but it can start to inhibit the chemical reaction inside the battery and lower its overall volts.

That drop won’t technically affect the battery’s capacity but it will make it seem like it has less charge. So, the devices you’re powering with the battery will behave as if the battery had a low charge rate or there was an issue with its connectors.

What to do?

Naturally, when this happens, you can exactly reach inside the battery and clean the crystals off the electrodes. So, prevention is the name of the game here. The basic advice is to discharge the batteries completely before you get them back to full charge. There are two problems with this tip, however:

  • This means using batteries that are low on power for a while which can be annoying or impractical with certain devices
  • You also risk running into another problem – battery pole reversal – which can happen if you over-discharge your batteries. Most modern devices are Smart and protect their batteries from pole reversal but basic devices such as flashlights don’t have such fail safes.

So, the way to avoid both the pole reversal and voltage depression issues is to fully discharge your NiMH batteries but only in specific devices, and to then NiMH charge them properly.

How to properly use a NiMN charger?

As with any other electronic device, the first rule is to always follow the instructions in the device’s manual to the letter. With that out of the way, let’s go over a quick step-by-step guide for charging your batteries:

Use a Smart charger for all your RC NiMH batteries

Such a charger is able to monitor the condition and charging progress of your batteries, has a timer, prevents overcharging, and even uses less power than most other basic chargers. What’s more, quality Smart chargers aren’t as overpriced as they once were so there’s no reason not to get one.

nimh, battery, charging, batteries, lose

Don’t trickle charge but don’t overcharge either

The maximum capacity of any NiMH battery is 120% and you should never charge them above that. If you don’t have a Smart charger with a timer, make sure you time the charging time of your batteries appropriately.

The reverse problem is trickle charging – charging your battery too slowly or trickle charging at a rate lower than 0.5A. There are moments and situations in which that’s acceptable – we’ll mention them below – but, in principle, your battery should be charged at the appropriate pace for its capacity.

Don’t fast charge your batteries either

Every battery has a maximum current limit it shouldn’t go over – in most cases that’s the battery capacity divided by 1000. So, for a battery with a 3,000 mAh capacity, the maximum current limit would be 3.0A.

nimh, battery, charging, batteries, lose

Store your batteries properly

We’ll elaborate more on that below but adequate storage at room temperature and at proper humidity levels (~40%) is key for prolonging their lifespan as excessive air humidity and heat can drastically hamper your battery’s condition and longevity.

Get quality NiMH batteries in the first place

This should go without saying but quality matters with batteries as with everything else. Good NiMH rechargeable batteries will have a low self-discharge rate and won’t lose almost any power while they are sitting on the shelf, waiting to be used.

Consider cell packing if you have the time

A technique that includes trickle charging is “battery cell packing”. The principle here is simple – first, you trickle charge or slow charge your battery at 1A current until the charger says it’s “full”. With slow charging, however, a battery is never really full, so, remove the battery from the charger and let it rest overnight.

Then, plug the battery in the charger again the next day and charge it at full current (1/1000 of its capacity) for a little while – 15 to 20 minutes or until the battery starts getting warm.

nimh, battery, charging, batteries, lose

This technique “packs” the battery cell and makes sure your recharged batteries are always at full capacity. It’s worth saying that, while some people swear by this method, its actual effectiveness is questionable and many would say that the same result can be achieved if you just put a quality NiMH battery in a quality Smart charger at full current for the appropriate amount of time.

How to store NiMH batteries to extend their lifespan?

Proper storage is another key aspect if you want to keep your RC NiMH battery out of the recycling bin for as long as possible. Here are the basics to keep in mind:

Maintain proper conditions

When you store your NiMH batteries you need to make sure they are not in direct sunlight, they are kept at room temperature at all times (so, not near heaters or in cabinets near hot water pipes), and the humidity in the storage area isn’t too high (around 40% and not above 50% if you can help it). Obviously, they should also be out of the reach of young children.

Recharge your batteries periodically

This doesn’t apply to short storage intervals but if we are talking about multiple months and years, it’s a good idea to check your batteries’ level every once in a while and recharge them if they’ve started to drop below 20%.

The reason is simple – letting batteries stay undercharged for long periods of time is known to slowly decrease their maximum capacity.

Consider cycling your batteries occasionally

This is a good tip for really long storage periods such as multiple years. The idea is to slow charge your batteries at 0.5 or 1A to full, then slowly discharge them down to 60% and repeat the process a couple more times.

The idea behind this is to keep the electrolytes inside the battery from settling during long periods of inactivity. So, while slow charging isn’t recommended when actively using the battery, it’s not a bad idea when you need to cycle it to keep it alive.

In conclusion – how to charge RC NiMH batteries?

The simplest way to summarize all of the above is – get a good Smart charger and follow its manual as well as the instructions on the battery pack itself. Don’t overcharge your batteries but don’t leave them undercharged either. Discharge them fully when you can but don’t over-discharge them either.

In short – charging your RC NiMH batteries isn’t all that complicated but it should be done properly and with care if you want to prolong their life cycle and use them for many years to come.


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