Garmin - Batteries and Satnavs

Batteries, Power -

Garmin - Batteries and Satnavs

Batteries and Satnav’s

The question ‘Which battery is best battery’ will be asked by almost everyone using a portable Satnav, making for a great subject to deep dive into Garmin - Batteries and SatNavs.

This is what I found out after my Satnav ‘ran out of battery’ and I got lost on a long cycle ride in the lanes of Devon and my body ‘Bonked’.  Your body runs out of glycogen and you physically can’t go any further.

Batteries, the Chemistry- List 

There are several kinds of chemistry in the AA size and they can be divided up into their ‘basic’ chemistry type, some, but not all, of which are:

1. Zinc-carbon

2. Alkaline

3. Metal

4. Lithium

Each has its place in different appliances, and the question becomes one of Price versus Capacity (amount of energy the battery can supply to your device).

Now my problem occurred because I ‘Ran out of battery’ on my ride. This occurred because I was to eager to “get going” so I shoved in a couple of fresh Alkaline AA batteries from a new pack, told my Satnav I was using alkaline batteries and off I went.

Batteries - Capacity – Satnav’s need Battery Capacity

One of the things your Satnav needs is the biggest capacity AA battery you can get (or whatever size your Satnav uses). AND the second thing is a battery chemistry that will deliver that capacity as the Satnav requires.

Now I knew I needed capacity and my Alkaline AA’s had about 1600mAh. But I did not know about the battery chemistry. 

Side note on what mAh means

A 1600 mAh battery can give you 1600 mA for one hour – in theory. Or 800mA for 2 hours, or 16mA for 10 hours… and so on. As the ‘Draw’ on capacity goes down the time it will last goes up.

But it’s not that easy – there is the Rated Capacity AND the Battery Capacity. The rated capacity is what you see printed on the packet – its what the battery manufacturer claims their battery has; it’s not what the battery actually has available to the user. The capacity available to the user is Lower than the rated capacity. The Rated Capacity being found in a temperature-controlled lab, with a cell which has just been made and is ‘Switched on’ (see closing comments) with a full charge. Not in the supply chain for a number of months, then sat on a shelf, all the while self-discharging.

Now we will discount the Zinc-Carbon straight off – They Don’t have near enough capacity. (google search – between 400~900 mAh)

So, my 1600mAh rated capacity Alkaline has in fact less than that available for use.

Back to battery chemistry - Alkaline batteries suffer from a problem, for us, the Satnav users. They don’t like giving up their capacity quickly (high current draw), if the Satnav puts a high demand on the battery they will ‘run out of battery quickly’.

So Alkaline batteries are not that good in a Satnav. Thus my problem occurred as I had a low capacity Alkaline battery, and my Satnav was drawing to much from them (I had the map on continuously, high demand by the Satnav for current, until battery failure) 

Lithium chemistry batteries

Basically, they are too expensive for rechargeable AA cells, and the Lithium-ion type can react violently to overcharging – exploding. They are 1.5v as opposed to the 1.2v not sure if my Satnav will tolerate the voltage difference – I suspect it will but I’m not going to be the one putting it to the test.

This left me with the remaining choice of the metal chemistry batteries from the list, as a source of power for my Satnav.

Nickel chemistry batteries 

There are several common types 

Nickel Cadmium  NiCad - produces very poisonous products when recycling

Nickel Metal Hydride NiMH – Much safer to recycle

Nickle Iron NiFe – Expensive compared to the NiMH

So, I went for the nickel metal hydride batteries which are readily available online.

Nickel metal hydride batteries Ni-MH (NiMH)

Having settled on NiMH do they fulfil the requirements

Hight Capacity – yes I found them up to 2700 mAh but at this rated capacity can cost up to £26.95 for a pack of 4 (at the time of writing – 12/08/20). So settled on eneloop pro 2500mAh. And the chemistry will stand high demand from the Satnav – so good choice, right?

Um... not quite, they need to be charged in a specific way.  They are intolerant of overcharging. (Not like the Li-ion which can explode) their life is reduced by overcharging so that claimed 500 recharges on the packet will not happened if you overcharge them.

Side note on life cycles v Capacity

To obtain the high capacity batteries the manufactures ‘Dope’ (add chemicals) to the basic battery chemistry. This is at the expense of life cycles. A 1000mAh battery can be recharged many more times than a 2500mAh battery. So, check before you purchase your batteries on what you are getting – Very high capacity can only be recharged a few hundred times.

Further you will not get 100% rated capacity after recharging. Typical figures are 66%~93% of rated capacity.

Your 2500mAh on recharging will be between 1650mAh and 2325mAh. You pay for what you get – the more expensive makes will be at the higher end of the scale. Cheap rechargeable’ s will be lucky to make 66% rated capacity on recharge.

To maintain the life (number of recharge cycles) you need a charger that is built for NiMH batteries and will detect “negative delta V”.

What this means it that when the battery reaches its maximum charge the output voltage drops slightly (it’s a small drop) and your charger needs to be able to detect this and stop the charger. It’s no-good using any-old AA battery recharger and expecting to get the best out of these batteries – it won’t happen.

At a pinch you can use any old AA battery recharger but will significantly shorten the life of the battery, and worse case cause the battery to leak by overheating it. Some cheap chargers only detect the temperature rise of the cell – its comparatively easy to do electronically; but by that time you have caused cell damage and are going down the road of shortening the batteries life, and causing a permanent capacity drop.

Following my research, I settled on a Maha Powerex MH-C9000 analyser/charger. As we are heavy users of batteries, Satnav, clocks, wireless phone, wireless mouse, keyboard, smoke alarms, remote controls, etc.

Other makes of charger I considered were:  La Crosse Technology BC1000, Nitecore D4, Opus BT-C700. But there are A LOT of lookalikes for these chargers from the far East.

It’s worth noting that I just purchased a tool (for my bike workshop) from abroad and got stuck with customs and post office handling charges (no more European union). Which while cheaper online cost me more in the end! 

Buyer beware – there are a lot of fake/look alike chargers out there (especially on Amazon). If you are going to use NiMH, unless your charger specifically says it uses “negative delta V” sensing, take care, it could be using thermal sensing. 

Some other things about NiMH batteries

They self-discharge, so fresh out of the packet will not be ‘full’ it can be as much as 30% per month. Some brands have a chemistry which combats this effect, eneloop being one of them.

If stored for a while they need to be ‘switched-on’ before they will work optimally, it’s a form of discharge/charge cycle some chargers call it breaking in. 

Two batteries used together in the Satnav will work to the capacity of the weaker cell. So, using your charger/analyser to match up the 2 cells will give you a stronger pair and a weaker pair from a pack of 4. Remember it is very unlikely you will get the capacity stated on the packet.

Some good news is they can be rapidly charged. A good charger can charge them within 2 hours.

NiMH are temperature sensitive they don’t like getting to hot or to cold. Don’t leave them in direct sun light. Performance will drop in cold/freezing weather (Scottish mountains in the winter could see a 50% capacity drop. Keep it under clothing and warm)

Charging them – if they get hot you have the wrong charger; they should only ever get warm.

Kelvin Phillips

Further reading

1. Best batteries for a Garmin GPS device

2. Power packs for Outdoor GPS units

3. Charging a GPS watch from a power pack - how long does it take?

4. Power and batteries - GPS Store


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