The care and feeding of your Glider Batteries!

Battery under test

There's a lot of misconceptions about the source of electrical systems failures in gliders. The most common cause is a crook battery.
Here's the basic facts about the common glider battery and how (or how not!) to look after it.

This document deals with the typical "12V, 7 amp-hour gell cell" type battery. It's also known as a sealed lead acid or SLA battery. This is the most common type in use to my knowledge. Basically it's a lead-acid battery, similar to what's in your car but a lot smaller and fully sealed so you can use it in any position. It's also a lot smaller and designed for smaller currents and easier charging. In normal use it will last many years, certainly a lot longer than your car battery. However if abused it will die in a matter of weeks.

The two easiest ways of killing your battery are overcharging (or poor charge control), and over discharging.


Overcharging could be caused by the battery being charged each day or each week when in reality (assuming no transponder use) a fully charged 7AH battery could last up to 10 days normal use in a glider with modern avionics. This overcharging is bound to happen if the charger is a simple high current charger.

In a typical unattended situation (i.e. most clubs!) the best charger is the "constant voltage" type. This charger has an open-circuit output voltage of about 13.8 volts. Connect the (discharged) battery and the charger supplies a moderate current that decreases to zero as the battery terminal voltage rises to fully charged (about that 13.8V). The battery can be left for weeks with no harm being done.
Alternative chargers are the "trickle charger", or a simple charger on a manual timer. Both of these may be OK if you are sensible when using them. Neither are a good idea in a club situation.
A suitable charger would be model M9531 from Dick Smith

Never use a cheap "car battery" type charger.
New to the market are "intelligent" chargers that sense voltage rise and are able to fully charge the battery quickly and efficiently. I'm sure one of these would be a good choice too.

Over Discharging

What I really mean here is leaving the master switch on (plus the radio/vario etc...). This is a typical club scenario...finish flying, rush to poke the glider away in hanger and have a beer. Everybody thinks someone else checked the master or removed the battery. It's easy to say but simply the best thing to do at the end of the day is unplug the battery.

The sad news is that once the unloaded terminal voltage drops below about 12V damage begins. The battery "sulphates" and looses it's capacity. As the voltage falls further even more damage is done, especially if it remains discharged for a period. The battery is irretrievable now.

Now the problem is this: The battery is put on charge. It may appear to take charge as per normal. Terminal voltage rises as is should, the appearance is of a fully charged "good-un". However once installed in the glider it will soon die, most likely the radio dies on transmit (but still receives!). All the experts blame the radio! My best advice is bin the battery and start again.

Test the battery

If you are still unconvinced try a load test: Get a 20-30W car lamp (or a large 6 ohm resistor). This draws about 2A. Measure the terminal voltage as the battery discharges. If you get an hour or so before in falls below 12V then at least it has some capacity. Note that at this high discharge rate you won't get full 7AH capacity. Even a new battery would struggle to give 2 hours.

Some other facts

Battery voltage decreases with falling temperature. In cold conditions (wave!) the battery will perform worse.
An unloaded terminal voltage (i.e. battery on it's own) of 12.5V indicates a battery of about half charge (but beware, half charge of a crook battery isn't much!)
Most good Glider radios will still transmit down to 10.5-11V. This is the sort of voltage they will see once your battery falls to about 12V. Internal resistance in the battery, and the resistance of your glider's fuses and circuit breakers means that considerable voltage is lost because of the high current drawn on transmit.
Some radios are worse than others. GA (powerplane) radios usually see 14V or more due to the alternator charging the battery. Thus some GA radios are no good in gliders.
Transponders are the most power-hungry device in your glider. Consider dual batteries if you have a transponder fitted. Gel cells have a low self discharge rate. If stored fully charged they are still good to go after several months. Don't store them in a discharged state.
You can buy your battery direct from the dealer Exide Technologies.

Contact them at 0800651611 or

You'll pay about NZ$21 and get a fresh item. Buy in a sports store or autoelectrical shop and you'll pay $50-60 and the battery may be a year or two old. Freight is the big killer if just buying a single item. Ask about the club and get a bulk order, you'll save on the courier cost.

Recently (2007) I note Exide's prices have gone up from $13 to $21. However I see that Soanar have them a lot cheaper (a good brand too). Have a look at: model number SB2486. They have some good chargers too.

Other Battery types

Weight for weight gel cells store less energy than other rechargeable battery types. NiCad and Nimh could be another choice if more capacity is required. However NiCad are now best left for high current devices such as electric drills where they still reign supreme.
The more modern Nimh have huge energy density (witness the 2.2AH AA cells about now) are a possible alternative to SLA. You'd probably have to make up your own "pack" to replace the original SLA. Check the weight of the completed product if its in the nose or tail. You'd need strings of 10 cells, maybe 11 if you've got one of those GA radios. Nimh have a high self-discharge rate, about 1% a day. The issue here is to recharge if they've been left for a few weeks. Both Nicad and Nimh need careful charging. It's best to spend a good amount on money on this as the battery cost is much higher than SLA types. Oh by the way, have you noticed just how quickly your cheap electric drill/screwdriver etc batteries die? Usually this is all due to very crude charging circuits and overcharging. It's cheaper just to by another cheap drill...
Lithium Ion are at present too expensive and fragile.
Despite the alternatives mentioned above I somehow think the SLA will be around for a long while yet!

JR 2004