Hitchcocks Forum

A question for all the electrical geniuses out there..


I'm running Thumper on a 12v/5ah battery at the moment. I've noticed that my GPS appears to draw too much and closes down after minutes of operation on the road.


I wondered if uprating the battery to 12v/6ah (or even a higher ah rating) would help solve this.


While we're on the subject, what is the maximum rating one could apply? I've seen a battery at 12v/12ah (144 Watts).

Could be time for an alternator upgrade if you're still using the original Indian item. Fitting a slightly larger battery won't help that much if all your electrical needs are greater than what the alternator can supply. If you fit the battery tray off an E/S model you could fit a 12AH or even 14AH battery, but that only hides rather than addresses the problem. Our hosts sell upgrades for the four-wire AC/DC alternators or Lucas RM21 type for an all DC system, otherwise go to someone like AO Services and treat yourself to a three phase alternator with some serious output. www.AOservices.co.uk A.

I suspect it may be the condition of the battery that is the problem here. I know things like GPS require a constant voltage and it may be that it is switching off as the voltage drops below 12. Other systems on the bike (except electronic ignition systems) will be far less fussy about voltage. I would check the voltage with a meter while under load.

I suspect Pete is on the right track. Surely a 5ah battery would be sufficient to power the device for a long time especially if it was receiving charge at the same time. The current set up should be more than adequate unless instead of GPS you mean some ancient valve operated directional radar set!.

Perhaps I should have been more specific. The battery is a maintenance free job, bought less than two years ago. The gps is a new Tomtom Rider, christmas 2014 present from the long haired part of the outfit.


The alternator upgrade idea looks good. I might be prepared to do that...

My ancient sat nav will work on it's internal battery for at least 30mins before it shuts down, and that is with no charge going into it. I cannot imagine it is a problem with battery capacity or lack of charge output from the alternator. Another possibility is that the output voltage and charge rate is too high and the GPS is shutting down as a consequence. I would measure the voltage at the connection point and see what's going on before going to a lot of expense on upgrades.

OK, Exile, I notice that you have a basic misunderstanding of what 'Ah' means. I know this because you wrote 12v/12ah (144 Watts), which doesn't make sense.



There is a difference between amps (A) and amp-hours (Ah). When we are talking about amps, you are correct that you can multiply the amps and the volts to get the watts (watts being a measure of the power being dissipated at an instant in time):



watts = amps x volts



amps = watts / volts



volts = watts / amps



For example, a 60 watt headlight bulb draws: 60W / 12V = 5A



However, none of this is relevant to what you want to know, which is about amp-hours (Ah). This refers to the electrical capacity of the battery, and it is easier to give you some examples.



Your battery is rated at 5 amp-hours (5Ah), which means it will supply 5 amps for 1 hour. If you draw only 2.5 amps, then it will last for 2 hours. If you draw 1 amp, it will last for 5 hours. If you draw 10 amps, it will last for 0.5 hours.



Hopefully you can see that if you multiply the current by the time, it always comes out to the number 5. I'm hoping this feels intuitive for you: if you take out twice as much current, it will only last half as long. Thus, if I had a 10 amp-hour battery, it would supply 10 amps for 1 hour, or 1 amp for 10 hours.(*) Of course, all these numbers assume you are starting with a fully charged battery.



Thus, you are partly right in thinking that a fully charged 6Ah battery will drive your GPS for a bit longer than a fully charged 5Ah battery, but that isn't important because your battery should not be driving the GPS. When your motorcycle is running, your battery "floats", neither providing nor drawing current (apart from a bit to charge it up). The electrical equipment should be powered by the alternator.



If your alternator isn't powerful enough to drive your GPS in addition to everything else (ignition, lights, etc), then - as you would imagine - the battery steps in to make up the difference for as long as it can before it goes flat. If this happens you have a problem with the alternator (or voltage regulator), because the alternator should easily have enough headroom to operate the GPS on top of everything else.



HOWEVER - are you SURE the GPS is closing down because of a low supply voltage? Something makes me think there is more going on. Is it programmed to shut down after a certain amount of time without the screen being touched? Are you SURE it is actually being powered by the bike's 12V supply, and not actually running on its internal battery?



I bet this is something to do with the GPS itself, and not the alternator, regulator or battery.

(*) The experts will know that I've slightly simplified the amp-hour explanation by not including hour rate (e.g. 5Ah at 10 hours), because it only leads to confusion at this stage.

Exile, is your GPS getting vibration? They don't like it up them, especially if taped to the handle bars!



you bullet should produce at least 5 amps of current. By the time the ignition has scoffed a little, there should just be enough left over to run your GPS and a small 12V camping kettle!



Maybe if you run with the headlamp on, that might produce the problems of shutting off. Will think on it a bit more...

Tom Tom draws about 1/2 / .5 amp, Does the bike battery charge the battery in the Tom Tom?

If the ammeter reads in the middle or to the right all is good, charge capacity is greater or equal to load. If it reads to the negative then bad, load is greater than charge rate which would be a pity because you will need the GPS to get your location for the breakdown people because the bike itself would breakdown soon after.