The care and feeding of your Glider Batteries!
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
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
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.
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 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
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
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
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!