48 volt battery charger

[DPW666]

Hi,

I didn't know where to put this but a charger is solid state electronics so move the thread if it is not apropriate here. I'm building my electric bicycle based on a 48 volt system and now realised that charging each batteries one at a time at 12 volt is alot of hassle. I want to build a 48volt one, but all the current sensing chips to switch the charging in trickle mode I can find are made for 12 or 24 volts. Does anyone know of one that can do 48 volts?

Thanks for any help!

[JBL]

IIRC You can't charge batteries in series.
Eache one need to be charged separatly or at least in parallel with a resistor in series with each batterie. Depend on the type of charger.

[DPW666]

Are you sure? All electric bike or scooters have their batteries in series (2x12, 3x12 or 4x12) and are charged with the same voltage charger. I found some selling but they are made to charge the batteries from electric golfcarts so their current is way to high. All I need is about 2 amps nothing more.

[Brian Donaldson]

There's no problems charging batteries in series if they are all the same type, size, age, specific gravity, etc.(matched set that are always in series when charged and discharged. An occational long term slow trickle charge is good to top off all cells.

If the circut you plan to use senses current (in series with battery), then the voltage will not matter. If it senses voltage, then a voltage divider network could scale the voltage by 2 or 4.

All battery packs greater than 2 volts (except lithium) are cells in series. (12v cordless tools use 10 1.2v nicads or NMH... auto batteries are 6 cells 2v lead/sulfuric acid etc.)

[DPW666]

well thats the problem, I can't find any circuits that charges at 48 volts. The higher I found is 24 volts. If anyone knows where to find one please tell me cause I can't find any.

[Circlotron]

You don't really need to switch over to trickle charge mode as such. Presuming you are using sealed lead-acid batteries, just make yourself a supply that puts out 54.6 volts (=13.65 volts per 12v batt) with a 2 amp charging current limit. The batteries will draw 2 amps on charge until their voltage rises to 54.6 total and then the current will gradually reduce to an eventual low trickle value while the voltage remains at 54.6 for as long as you like with no problem. e.g. years. An even better way would be to charge each 12v block separately but at the same time, using 4 chargesr(!) This would help to ensure that each cell in the string would get an equal voltage and therefore charge. The longer the string of cells the more likely they will get unbalanced over a period of time.

/Circlotron - mucks around at work with telephone exchange type battery chargers. 54.6 volt, 585 amps rack mounted switchmode.

[Brian Donaldson]

What type of barreries do you plan to use. Different ones need different types of voltage or current regulation. Circlotron's comments are a great for lead acid batteries and would be quite simple. Nicads are different. I think they are best charged in short high current pulses. This is not so simple. They can be trickle charged, but don't overcharge. If you know the ampehour rating, you can do the math. 1.2 amp hour battery will nned to slow charge at 120 ma for 10 hours (for ideal battery) to 10.5 hours(real battery)

[DPW666]

well I am using 4 12v 7amp lead-acid batteries in series. So you think that just a normal power supply at the good voltage could do the thing? I always thought that letting a non-automatic charger always plugged could overcharge the battery.

[subwo1]

Well, automobiles have used alternators which hold the battery at a somewhat constant voltage. If you were to do as suggested and make one to apply about 13.5 volts per battery in series with occasional trickle charges to even out the charge on each of the 24 individual cells, then you should be OK.

You may consider using a SMPS circuit for compactness. Then you can consider building the charger onto the bicycle. You may be able to base it on my circuit which is described at

http://groups.yahoo.com/group/switchmode

[Pjotr]

Well it is not that simple. With lead acid batteries the trickle voltage depends of their use. Is it cyclic use or is it standby use. For standby use a trickle voltage of 2.35V/cell is recommended, for cyclic use 2.45V/cell is recommended. So the best and fastest way is to charge with 2.45V current limited to app. 0.3x Ah value. When the charging current drops to 5% of the Ah value switch over to 2.35V to trickle charge. Besides this, the voltages are also slightly temperature dependent. Sensing current can be done with a resistor in the ground lead of the batteries. But the perfect solution is a compensated current transformer from LEM. These will measure DC and are not that expensive, around $10. For instance the LTS-NP25 (PDF at http://www.lem.com/ ).

But for your bicycle a fixed voltage of 2.45V/cel will suffice. Provided you do not leave it at the charger for days.

More important with lead acid batteries: Do not leave them partially discharged for extended periods and do not discharge them fully. This will greatly reduce their life. After each use recharge them as soon as possible.