# Calculation of power for battery charging

1. Sep 12, 2008

### AP1

Hi - I am trying to calculate the power required to charge a battery in order to compare the energy requirements of an electric scooter versus an internal combustion scooter. The electrical scooter specifications state:

battery: 12V/50AH x 4
time to 90% charge: 4.5 - 6 hours
charger: 400W / 7A

Am I correct in assuming that to recharge a discharged battery to 90%, it would take 2.4 kWh (i.e. 400 W x 6 h)?

Thanks for any advice you can offer.

AP

2. Sep 13, 2008

I think when you recharge a battery it is only about 50% efficient. I think your calculation is right.

3. Sep 13, 2008

### Staff: Mentor

I always thought battery chargers were more efficient than that. In any case, your numbers give 135W for the battery and 400 for the charger, an efficiency of 34%.

When someone asks me a question that involves charging efficiency (such as evaluating an electric vehicle), I typically use something like 90%. But I don't really have any basis for that.

4. Sep 13, 2008

### OmCheeto

I guess we have to assume you are charging a lead acid battery.

Given that assumption, your assumption is not correct.

A good charger will have 2 stages:
1. Maximum current with a rising voltage
2. Maximum voltage with a descending current

So the charger will not be operating at the rated power and current over the full charge.

Without having a http://www.thinkgeek.com/gadgets/travelpower/7657/", you can estimate the power transferred to the battery by graphing the voltage and current about every 30 minutes.

Last edited by a moderator: Apr 23, 2017
5. Sep 13, 2008

### Redbelly98

Staff Emeritus
6. Sep 15, 2008

### AP1

Thanks for all of the replies. The problem I have is that I do not have the batteries and charger to actually measure the power demand during a recharge cycle. I am trying to estimate this so that I can calculate (again, an estimation) the equivalent CO2 emission of electricity generation versus the same for a gasoline engine. I already know the CO2 emissions per kWh for the local electricity supply and so need to estimate the kWh required to charge the e-scooter battery.

Best regards,

AP

7. Sep 22, 2008

### Pumblechook

Chargers might reach 86% efficient but the charging process is a similar figure and then there are losses in the vehicle particular if 3 phase AC motors are used.

You must not discharge a battery too much otherwise it wont last long. They has been a debate what the Ah means?? Real useful power without demaging the battery or a 'theoretical' figure if the battery was 100% discharged.

Then there are transmission losses from the power station of 7 - 10%..

Long chains involving energy conversions add up to large overall losses.