# Charging a cycle battery with an inverter?



## richardsnow (Dec 9, 2007)

I have a 110amp leisure battery fitted to my motorhome which is charged (apart from when on mains) by the engine when travelling and supplemented by a 60watt solar panel during daylight hours.

I have just brought an electric bike and need to charge the 36volt battery from within the motorhome when no electric hook up is available, but I am at a loss to know how much drain it is using.

My 300w inverter works fine. I then plug the mains lead of the cycle battery charger into it and it starts to charge ok but is quite a drain when the engine is not running.

The following info is printed on the charger -

Li-ion Battery CHarger
Input AC 100v - 240v~1.8A Max
Output 42.0v 1.8A

Can anyone help with the following info please-
a) How many amps will this draw from the 12v battery when charging?
b) How long will a fully charged 110amp battery last with that draw?
c) Alternatively would it be better to only charge it when the engine is running?
d) If i run the battery dowm how long would it take for the engine to recharge it again?

Sorry the question is a bit long but the whole thing is a bit complicated for me!


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## ched999uk (Jan 31, 2011)

The numbers seem a little strange. watts = amps x voltage. So at mains 1.8x230 = 414 watts. The output is 1.8 x42 = 76watts. 

The best way to determine how much current the inverter is using from your batter is to measure the current via a current meter (beware most multimeters don't go above 20amps = 20x12 = 240 watts).

Even if you know what the charger pulls you dont know how efficient the inverter is.

How long does the bike battery take to charge from fully flat?

I would suggest you can get 55amps out of a good 110ah battery!

So a 300watt inverter running for 1 hr might use 25amps from your battery IF your inverter is 100% efficient. My guess is that a 300watt inverter would use more like 28amps from your battery.

I would suggest charging while the engine is running.

A completely flat leisure battery may take 3-4 hrs to fully recharge from the alternator but the alternator isn't great at achieving max capacity from your battery. The 240v charger is usually better at giving max charge.

Hope that gives you some ideas.

I am no expert on batteries, chargers, or inverters the above is just info I have picked up on the way


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## sideways (Jun 2, 2008)

I dont understand the calculations but can tell you that i have two 110 amp leaisure batterys and a 300 watt inverter and i cant charge two e bike batterys with out flattening the leisure batteries, i resorted to charging from the inverter while driving, no good if your staying in one place.


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## richardsnow (Dec 9, 2007)

I thought that this might be the case. Can you give me any idea how long a drive you require to charge one of the bike batteries please?



sideways said:


> I dont understand the calculations but can tell you that i have two 110 amp leaisure batterys and a 300 watt inverter and i cant charge two e bike batterys with out flattening the leisure batteries, i resorted to charging from the inverter while driving, no good if your staying in one place.


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## GerryD (Sep 20, 2007)

I manage to charge ours (24V batteries) using a 150W inverter. They only need charging every couple of days so I charge one while we are traveling and the other whilst we are stopped for the night.
Gerry


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## pete_b (Jul 13, 2010)

Is it not about time they made these electric bikes to recharge when going down hill?
or do the new ones do this?


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## sideways (Jun 2, 2008)

if the bike batter is well down it takes 3-4 hours to charge while deriving.


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## richardsnow (Dec 9, 2007)

They already do, but the amount of charge gained is VERY small I understand. If the technology was good we could even have perpetual motion - use power uphill and replace it going downhill!!!



pete_b said:


> Is it not about time they made these electric bikes to recharge when going down hill?
> or do the new ones do this?


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## CliveMott (Mar 10, 2008)

It will take at least 7.5 amps from the 12v battery if both inverter and charger efficiencies are good. So three hours and you will have taken at least 22.5 ampere hours.

C.


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## richardsnow (Dec 9, 2007)

CliveMott said:


> It will take at least 7.5 amps from the 12v battery if both inverter and charger efficiencies are good. So three hours and you will have taken at least 22.5 ampere hours.
> 
> C.


Sounds about right thank you but how do come to the figure of 7,5 AmpH in the first place please?


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## CliveMott (Mar 10, 2008)

Very rough mental arithmetic!

You said

Output 42.0v 1.8A 

So I rounded that up to 48 volts at 1.8A to make maths easier and allow for inefficiencies. 
So divide the volts (48) by 4 and multiply the amps by 4 for the same power at 12 volts = 7.2 amps.

As you have two converters each with losses (inverter and then the charger) running one after the other I rounded up further to 7.5 amps (ish).

Bingo.

C.


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## richardsnow (Dec 9, 2007)

CliveMott said:


> Very rough mental arithmetic!
> 
> You said
> 
> ...


Great thanks for the explanation


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