can someone help with this situation

i have a 12v dc air pump which is 60 watt , i want to run it off a car battery in an emergency blackout to give O2 to my fish ...now i want to buy a battery to suit , so what is the calc to work out how long the battery will last using this 60watt pump ? also any ideas on what is a good way to keep the battery charged when not in use ? solar ?

thx in advance

The calculation is volts x amps = watts so your pump will use 5 amps.

Therefore if you purchase a 40 amp/hr battery then a full charge will give you 40/5 = 8 hours of running time.

Not sure the best charging method but a trickle charger may be the best option as the battery will likely be used infrequently.

Hope this helps.

CP49

+1 ^^^, you are however unlikely to get the full 40Ahr, aim at 75% of theoretical.

Use a trickle charger, solar is too unreliable and expensive.

thanks fellas , so what would u suggest as a better choice of battery ? someone mentioned deep cycle , or shall i just go on the amp/hr rating ...i use a battery tender on my Harley so i imagine i can find something similar to suit a car battery ....this pump is just an emergency unit , i will hook up a relay that once the mains drops out the 12v pump will start

If it's purely for occasional use a car battery will do the job just fine, if it's going to get regularly discharged then deep-cycle is the way to go.

The small trickle chargers are available at most battery shops and Ma and Pa stroes that sell bike batteries. Mine cost 350 baht.

As other poster have noted, 60w @ 12VDC = 5 amps.

Batteries are rated in amp hours (Ah), so you'll need 5Ah of battery power for every hour the pump needs to run.

What hasn't been mentioned yet is depth-of-discharge (DoD). For lead acid batteries, this is the key factor in battery life. Generally speaking, those who rely on battery power recommend going no lower than 50% DoD. Those who use batteries on a daily basis (e.g. off grid solar) usually aim for around 20-30% DoD in order to extend battery life. If it's purely standby usage and wont' see too many cycles, you could go as high as 80% DoD.

Based on your description it sounds like you won't be needing to use it on a routine basis, so 80% DoD should do. Working on that, for every 1 hour you need to run the pump you will want 6.25Ah of battery capacity.

As for how to recharge, a simple lead acid battery charger can be had for a few hundred Baht.

A solar system that could recover say 4-5 hours of usage in a single day (even if cloudy) would cost around 10K Baht (7-8K baht for a 280w mono crystal, 1.5-2K Baht for the solar charger, plus some wiring) as a DIY.

thanks for the replies ... ok have found a 120 Ahr battery ( dry cell) ... now i want to hook three pumps up off the one battery ... each 60watt ...my goal is to have them run for 4 hrs max in a major blackout ..so my calcs tell me this battery will do it with ease. The next issue is the three pumps are situated 3.5metres away from each other , my idea is to locate the centre pump with the battery and charger next to it ( this is also where the 220v power supply is) at this point i will install a contactor or relay to control the 12v pumps when the power cuts ... my question is this

1/ what size cable do i need from the contactor out to the other two pumps (3.5m each)

2/ do all battery chargers have their own cutouts when charged ?

thanks

1. I would run 2.5mm² 'speaker' cable, cheap and fine at 12V.

2. Cheap chargers use constant voltage to prevent overcharging, these are fine for car type batteries, the output is not however, very well controlled. Since you have a 'dry' battery you may want to use a 'smart' charger, the battery should have charge voltages marked on it, set the charger to the 'float' voltage for your battery.