Battery Charging time

[Pete]

Does battery charging (L-ion) time have a linear relationship with the state of the battery before charging. In other words, if it takes 4 hours normally to charge a depleted battery would it take 2 hours for a half charged one, 1 hour for a quarter discharged one etc.

Related to the above question, would a 300 Watt Inverter that I have in the car boot run a Li-on battery charger? , I often ride my normal bike at places I have driven to with the bike on the carrier. The idea being that if I went to one place for a ride and then drove to another one I could charge the battery on the way.

Yes, I know, electric bikes are supposed to get you out of the car! but I like to explore places on my bike that are just too far to pedal to, so is the use of an inverter a practical proposition? If so, other members might be interested, especially as they only cost about £25 - £30 for one that size.

Pete

 

[Straylight]

would a 300 Watt Inverter that I have in the car boot run a Li-on battery charger? Pete

Going by Ohm's law, P=I*V, where P= power measured in Watts, I= current measured in Amps, & V=voltage in volts. You're inverter, at 240volts has a maximum current delivery of 1.25Amps (I=P/V).

As most e-bike chargers are rated at around 2Amps, a 300w inverter would sadly not be up to the job. To run such a charger, it would need to be at least 480w.

I don't know how linear the charge times are, but I'd suspect not. My guess would be that the rate of charging slows down as the battery approaches full, but I could be completely wrong on this.

 

[Fecn]

Does battery charging (L-ion) time have a linear relationship with the state of the battery before charging. In other words, if it takes 4 hours normally to charge a depleted battery would it take 2 hours for a half charged one, 1 hour for a quarter discharged one etc.

Pretty much.. yes... except you have to discount the last hour from the equation.... E.g. if your battery takes 6 hours to charge then it's 20% per hour for 5 hours, and then a hour of mucking around at the end (Cell balancing) - The last hour of charging only accounts for about 5% extra energy stored.

Related to the above question, would a 300 Watt Inverter that I have in the car boot run a Li-on battery charger? , I often ride my normal bike at places I have driven to with the bike on the carrier. The idea being that if I went to one place for a ride and then drove to another one I could charge the battery on the way.
Pete

Yes - a 300w inverter should work just fine... but do remember that your car battery only has a limited supply of power too... A 36v 10Ah bike battery would use around 35-40Ah of car battery capacity to recharge... and many cars only have 65-90Ah batteries in the first place. Making sure the engine is running when charging is a good.. if somewhat fuel-inefficient... way to charge the bike.

 

[Fecn]

Giong by Ohm's law, P=I*V, where P= power measured in Watts, I= current measured in Amps, & V=voltage in volts. You're inverter, at 240volts has a maximum current delivery of 1.25Amps (I=P/V).

As most e-bike chargers are rated at around 2Amps, a 300w inverter would sadly not be up to the job. To run such a charger, it would need to be at least 480w.

You're muddling up your input and output amps there Straylight. A 2A bike charger is measuring the output amps... for example.. 29.4v@2A == 58.8 watts... The charger will be around 85% efficient meaning that a total of 69 watts is needed to power it. A 300w inverter would have no problem with that.

 

[Pete]

Thank you both. I did mean only when I was actually motoring Fecn. Sounds like it would be workable then.

Pete

 

[Straylight]

You're muddling up your input and output amps there Straylight. A 2A bike charger is measuring the output amps... for example.. 29.4v@2A == 58.8 watts... The charger will be around 85% efficient meaning that a total of 69 watts is needed to power it. A 300w inverter would have no problem with that.

The charger for my Wisper has an input current rating of 1.5A @ 240v, and an output of 3A @ 42v, so I guess it may depend on the charger concerned?

 

[flecc]

Does battery charging (L-ion) time have a linear relationship with the state of the battery before charging. In other words, if it takes 4 hours normally to charge a depleted battery would it take 2 hours for a half charged one, 1 hour for a quarter discharged one etc.

Even on some non-lithium types like NiMh without cell balancing functions, the latter stages of a charge can take longer for a given charge amount, the reason being rising cell resistance as the charge rises and temperature increases to the cutoff point.
.

 

[Pete]

Thanks Flecc, I had an idea in the back of my mind that this was the case.

Pete

 

[tillson]

The charger for my Wisper has an input current rating of 1.5A @ 240v, and an output of 3A @ 42v, so I guess it may depend on the charger concerned?

wouldn't that mean the charger consumes 360 W from the mains whilst giving an output of 126 W? An efficiency of about 30%

 

[flecc]

wouldn't that mean the charger consumes 360 W from the mains whilst giving an output of 126 W? An efficiency of about 30%

If it's one of the later Wisper chargers which are made by HighPower (HP), the same as those eZee use, they have an internal fan motor which will lose some current. However, the 1.5 amp rating on those is for 115 volts input, the input rating for 240 volts being 0.9 amps with an output of 2 amps.
.

 

[Fecn]

[QOUTE=Straylight]
The charger for my Wisper has an input current rating of 1.5A @ 240v, and an output of 3A @ 42v, so I guess it may depend on the charger concerned?

wouldn't that mean the charger consumes 360 W from the mains whilst giving an output of 126 W? An efficiency of about 30% [/QUOTE]

It's all about maximums.. not continuous. That 1.5A figure will refer to the inrush current when the charger is first attached to the mains supply and input capacitor fills up with charge. Once that has happened (in around a millisecond) then the current will drop down to around 0.6A. If you were fitting a fuse on the input, then anything less than 1.5A might blow. Similarly, the 3A at the output refers to the max.... The charger won't necessarily be delivering that all the time (such as when the battery is nearly full), but if you wanted to put a fuse on there.. anything less than 3A would blow. Most modern chargers are around 80-85% efficient, so if your charger does 3A@42V, then it's probably pulling about 150W from the mains.

 

[tillson]

Thanks Fecn, it's all clear to me now