Volts x Amps = Watts

Mikex

I recently posted about solar charging the battery on my electric golf trolley.

I need to have a better understanding of the subject.

Please could someone confirm if my following interpretations are correct.

All examples using 12 volt battery.

A 30 aH battery will take 10 hours to fully charge from flat using a 3 amp charger?

A 360 watt motor will flatten a fully charged 30 aH in 1 hour?

A 12 volt / 3 amp charger could also be regarded as a 36 watt charger?

I would need 100 x 3.6 watt solar panels to power my 360 watt motor?

A small trickle charger quoted at 1.5 watt would be the equivalent of one eighth of an amp and would take 240 hours to charge a flat 30 aH battery?

Sorry if this a bit elementary but it is a long time since I was at school and I paid very little attention.

Thanks

Mike

zax47

All those assumptions/calculations are essentially correct (not allowing for %age efficiency of solar cells or any precharge of battery), but it's Ah not aH!

A 125mA trickle charger is really a "keep it topped up" device, but yes it would really take 240 hours as you calculated.

Jonesya

As a rough estimate the figures are ok but in reality there's a few complications - the charging process isn't 100% efficient so will take longer, the 30Ah capacity will have been measured using a slow rate of discharge (normally over 20 hours) so you're draining the cell rapidly in an hour you'll get less than 30Ah out of it. With the solar cells the power output will probably be a maximum output which requires a high light level, in reality output will probably be a lot less.

More info on battery data below - see the Red Book or NP Manual:
http://www.yuasa-battery.co.uk/industrial/downloads.html

keith969

You can't make the assumtion that the charging time is a simple function of the battery Ah and charger currrent rating. Firstly the charger may not be a constant current charger - a simple charger for a lead-acid cell will have a rectifier and smoothing capacitor, so the current it supplies will be a function of the load resistance, which in the case of a battery will be non-linear wrt time. More complex chargers will regulate the current depending on the battery emf in some nonlinear fashion.

Secondly there will be losses both in the charger and the battery being charged.

Same goes with discharge - it gives you a rough idea of the time, but its hardly precise. As the battery ages the amount of charge it can hold will decrease, as anyone who's had a battery powered electric drill for a long time will know.

lfc321

the posters above are all correct. you won't get anything like 100% efficiency

Mikex wrote: »

A small trickle charger quoted at 1.5 watt would be the equivalent of one eighth of an amp and would take 240 hours to charge a flat 30 aH battery?


Mike

But you can get much bigger trickle chargers - e.g. this one is rated at 7.5W so would cut the charging time down considerably (will still take a long time, though - as others have said these types of chargers are designed to keep batteries topped-up, not really to recharge them from flat).

http://www.ecofreak.co.uk/products/23/137/Sunsei-SE500-Solar-Panel.html

ormus

as above.
the calcs are generally correct, but no electrical/electronic device is 100% efficient. you have to make allowances for the losses.

Mikex

Thanks all for your replies,

I am pleased that I have a basic understanding of the subject.

I did expect it would be pointed out that there might be losses in efficiency. I also expected someone to post something beyond my comprehension.

The battery sizes given and charger capacity were only examples but were chosen to simplify the maths.

The reality is that when I bought my electric trolley I thought I might like to attach a solar panel to the golf bag so some charge would be supplied to the battery during the game. As the output of a solar panel is largely dependant on its size it is difficult to see how I could provide meaningful supplementary charging as I am playing.

If anyone has any ideas (that won't cost a fortune) the battery is actually 22AH 12 volt and the motor is 230 watt but it has a variable speed control so I assume I will rarely use 230 watts. Additionally the trolley spends much of its time stationary while I am sratting about in the rough looking for my wifes balls.

The mains charger supplied with the trolley is actually 3.5 amp and seems to take about 1.5 x to charge as the time I have played. If I play for 2 hours it takes 3 hours to re charge the battery.

Look forward to more replies.

Mike