How long will my battery last between charges?
Please use this as a guide, the best way to find what suits your requirements is by your own experience.
"The old car battery behind the shed, under the flower pots or in the garage probably won't be up to the job especially if it was faulty when removed from the car and has been unused for several months or years!"
How long Between Charges
You can calculate how long between charges by dividing the total lamp Wattage by the voltage (12V). Times that by how many hours per day the lamp will be on, then divide the Amp-hour rating of your battery by that.
For example:
With 1 lamp of 8W divided by 12V = 0.6 Amps.
If the lamp is on for eight hours in every 24 (0.6A x 8 Hours) = 4.8 Amp Hours of battery life per day.
If you have a 40 Amp-hour battery, 40 divided by 4.8 = 8.3 days between charges.
Most small car batteries are rated at 30 to 40 Ah, so expect roughly 6 to 8 days use from each charge. The power required when the lamps are off is insignificant.
Batteries intended for caravan or emergency lighting may be more suitable typically 60 - 85 amp hour. As a general rule heavier batteries contain more lead and have a higher Amp-hour rating.
| Battery rating | Lamp Wattage | Lamp on Time | Time Between Charges |
|---|---|---|---|
| 30Ah | 8W | 8 Hours | 6 Days |
| 40Ah | 8W | 8 Hours | 8 Days |
| 60Ah | 8W | 8 Hours | 12.5 Days |
There is also a PP3 (6LF22 / 6LR61) 9 volt battery, that maintains the timer cycle for up to 20 hours when the external power supply is removed.
The PP3 (6LF22 / 6LR61) 9 volt should be renewed annually.
A Word about Batteries
Old Batteries
If left unused for long periods, lead acid batteries tend to deteriorate, the plates "sulphate" and dramatically reduces the Amp Hour rating. A Sulphated battery will reach a fully charged state quickly but will also flatten quickly effectively acting like a much smaller battery. Charging and fattening a sulphated battery may revive it a little but full capacity will never be achieved.
Modern car Batteries
Modern car batteries are designed to provide large amounts of current for a short period of time rather than the small current for a long period as per this application. Batteries intended for caravan or emergency lighting are more suitable. As a general rule physically heavier batteries tend to contain more lead and have a higher Ah rating.
ChargingCAUTION
When charging a lead acid cell you should be aware that explosive hydrogen gas is produced. Never smoke or use a naked flame near a charging battery and always turn off the charger before connecting or disconnecting it. When the cells start to bubble the battery is charged and only the electricity supply company benefit from leaving it longer.
Maintenance free batteries that have no holes to top up the electrolyte level should only be charged with a special charger, normal charging equipment will almost certainly cause permanent damage and possibly an explosion.
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