# Amps used per hour calculation ???

#### Forestboy

LIFE MEMBER
Cam someone remind me of the formula to calculate amps per hour used please .
Tried the search but no luck but difficult on a poxy phone.
Trying to work out what the chargers for the 2 leccie bikes will use charging off inverter.

#### scotjimland

##### Free Member
watts div. by volts = amps, or amps per hour.

for example.. if it's an 84 watt charger, divide by 12v .. = 7ah

the inverter also some power but for a rough calculation we can ignore it

OP

#### Forestboy

LIFE MEMBER
Thanks Jim thought it was bit wasn't confident.
So 230v ~ 90w. = 2.55 per hour x 2 chargers = 5.1 amps per hour while charging.
Correct ?

Last edited:

#### scotjimland

##### Free Member
Thanks Jim thought it was bit wasn't confident.
So 230v ~ 90amp = 2.55 per hour x 2 chargers = 5.1 amps per hour while charging.
Correct ?

err.. no.. can't be 230v at 90Amps ...

what is the wattage of the charger ?

OP

#### Forestboy

LIFE MEMBER
Sorry should have typed 90w not amps just edited original

#### jockaneezer

Bit of a thread hijack but here goes. Our charger, Kalkhoff says 2 amps at 240 volts, plugged it into our 600 watt inverter and looked at the onboard display. Battery ( 110 a/h ) was showing 12.8 volts, switched the charger on and you could watch the decimal volts disapear before your eyes, within a couple of minutes it had dropped to 12.3 volts. Knocked the charger off then a couple of hours later when we set off, I switched the charger back on, onboard display showed alternator/e box was putting in 25 amps which quickly dropped to 15 amps and stayed there for the two hour drive which had the bike battery back to a full charge, knocking the inverter off dropped the running amps to 3, fridge on 12 volts using it ? Hadn’t thought the charger would have drained the battery so fast but I’m sure someone will be along to explain the black magic involved. At least I know now to charge the bike batteries when driving or on hook up.

#### scotjimland

##### Free Member
Sorry should have typed 90w not amps just edited original

.. no probs

so you have 90w x 2 = 180 watt , div by 12 = 15amps.. or amps per hour.

#### scotjimland

##### Free Member
Bit of a thread hijack but here goes. Our charger, Kalkhoff says 2 amps at 240 volts, plugged it into our 600 watt inverter and looked at the onboard display. Battery ( 110 a/h ) was showing 12.8 volts, switched the charger on and you could watch the decimal volts disapear before your eyes, within a couple of minutes it had dropped to 12.3 volts. Knocked the charger off then a couple of hours later when we set off, I switched the charger back on, onboard display showed alternator/e box was putting in 25 amps which quickly dropped to 15 amps and stayed there for the two hour drive which had the bike battery back to a full charge, knocking the inverter off dropped the running amps to 3, fridge on 12 volts using it ? Hadn’t thought the charger would have drained the battery so fast but I’m sure someone will be along to explain the black magic involved. At least I know now to charge the bike batteries when driving or on hook up.

here is why ...
2 amps at 240 v = 480 watts. so when charging from battery via inverter it will be using 480 div. by 12 = 40amps ..or 40ah
that's a an 85ah battery effectively flat in one hour

OP

#### Forestboy

LIFE MEMBER
Ok thanks but why divide by 12 that seems to be where Im going wrong as the inverter is putting out 230 that's what divided by.
Just plugged it in, panel was showing no charge on solar as batteries full. Now showing 3.5 amps going in with 1 charger running .

#### bungy

##### Free Member
Need to look at the whole thing
12v Leisure battery is powering the inverter which in turn is powering the charger for the bike batteries, so the two end points are the 12v leisure batteries and the bike batteries.

For the bike batteries chargers to receive 90w each to charge the bike batteries, the start point will have to deliver enough juice from the leisure batteries to drive the inverter to drive the leccie bike batteries charger

So...90w(charger wattage)/12v(the real power source) = 7.5 amps of power needed per charger from the source - so 15amps in total per hour for the 2

#### hilldweller

LIFE MEMBER
I'll muddy the waters. Watts is a wonderful unit, it's the real ENERGY used.

Problem is they don't rate batteries in watts, but you can.

You have 400Ah in batteries at 12V so you have 4800 watt hours.

Your bikes can be treated the same, mine is 24V 20Ah so 480 watt hours.

So you can immediately see that my 480 battery is one tenth of your leisure battery.

Put another way, your batteries are 4800 gallons of energy, my bike battery is 480 gallons so look how easy it is to picture filling my battery from your batteries.

So what are the volts and amps of your bike batteries ?

OP

#### Forestboy

LIFE MEMBER
I understand now I was converting to 230v instead of using 12v from the batteries.
Brian bike batts are 36v x 10 amps

By the way battery put on charge thro inverter at 12 30 fully charged at 2.05 solar now showing less than 1 amp going in and leisure batts back up to full. So solar panels kept up with the charging easily.
Don't know what size they are can't find label on them but 2 panels each 45 in x 21 in x 2.5 in

#### hilldweller

LIFE MEMBER
Brian bike batts are 36v x 10 amps

So you bikes hold 360 watts. Your battery bank 4800 watts. It's only approximate but easier to picture.

No use playing with numbers on solar, the available light is too variable. If they work they are big enough.

OP

#### Forestboy

LIFE MEMBER
Ok thanks all for the help and advice.

#### haganap

LIFE MEMBER
jeesus, although I found this thread helpful, Im glad Im a runner and don't need batteries for push bikes!!!

OP

#### Forestboy

LIFE MEMBER
jeesus, although I found this thread helpful, Im glad Im a runner and don't need batteries for push bikes!!!

You'll get old one day.

At least I know I'm still totally self sufficient. No point having the bikes and needing ehu to charge em.

#### Wildman

##### Free Member
The main point to remember is only charge the bike batteries when driving unless you have plenty of solar and sunshine to drive them.

OP

#### Forestboy

LIFE MEMBER
Charged them both twice over the weekend off the inverter and solar no problem plenty of amps left in batteries.

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