battery charger

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jap2525

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This may not be the place to ask this but I thought I would ask it anyway.
If you have a 48 volt ((8) 6volt batteries in series) system for a scizzor lift,do you have to have a 48 volt charger to charge the batteries or will a 12v charger put some charge on the batteries but not as fast? If you parrallel (2) 12v batteries for double capacity will a 12v battery charger still be sufficient to charge the two at the same time or would you have to disconnect them and charge them one at a time?if you have (2) 12v batteries in series for a 24v system do you need a 24v battery charger to do any good?
 
Re: battery charger

For your 1st and last questions (both similar questions in that they deal with batteries in series), in practice this probably wouldn't work. Most battery chargers have complex controls to prevent overheating the battery, prevent overcharging the battery, and to prevent charging the battery too fast.

In theory, (and for the simplest charger I can imagine), it might work depending on the design details of the charger.

Steve
 
Re: battery charger

For your second question:

If you parrallel (2) 12v batteries for double capacity will a 12v battery charger still be sufficient to charge the two at the same time or would you have to disconnect them and charge them one at a time?
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I believe most batteries are rarely (if ever) placed directly in parallel. The problem is that any small difference in the voltages of the two batteries results in a current flowing from one batttery to the other.

Diodes can be placed between the two positive terminals of the batteries to prevent this. (Resistors can also be used to limit the current flow - then one battery charges while the other one discharges until the voltages match). But this complicates the charging circuit. It would be necessary or desireable to connect the charger on the battery side of the diodes or resistors.

Steve
 
Re: battery charger

Originally posted by jap2525:
This may not be the place to ask this but I thought I would ask it anyway.
If you have a 48 volt ((8) 6volt batteries in series) system for a scizzor lift,do you have to have a 48 volt charger to charge the batteries or will a 12v charger put some charge on the batteries but not as fast? If you parrallel (2) 12v batteries for double capacity will a 12v battery charger still be sufficient to charge the two at the same time or would you have to disconnect them and charge them one at a time?if you have (2) 12v batteries in series for a 24v system do you need a 24v battery charger to do any good?
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a 12V battery charger will not charge a 48V battery stack. you might be able to get it to work if you tried charging two of the 6V batteries in series, but you can't use just any old charger.

there is no reason you cannot charge a battery stack of 2 12 volt batterys seperately. again you have to match the charger to the battery.

there is also no reason you could not (at least in theory) charge 2 12V batterys in parallel. however, no two batterys will ever have the same exact voltage output so putting them in parallel is not something done withoout taking some precautions. the same probelem will happen when you try to run them in parallel since the batterys will not be the smae voltage.

keep in mind that different battery technologys require different style of chargers as well for optimal battery life.
 
Re: battery charger

Here is the simplest battery charger, and one that might work for questions 1 and 2. (Batteries probably won't last too long if we leave them hooked up to this charger all the time!) Say we have a 12V car battery and a charger. We want to use it on our 48V forklift battery.

Take 120V AC from a receptacle, and connect a diode and a resistor and the battery to be charged in series. The diode allows only the positive peaks to pass with the peak voltage = 172 volts. The 172 volts is across the resistor and the battery. But we know the battery has about 12V across it. That leaves 160V across the resistor.

Using ohms law, I choose a resistor that gives me a reasonable charging current (say 2A for our car battery). 160V/2A = 80 ohms.

Now say I place this charger on our forklift battery. Now I have 172V-48V = 124V across the resistor. 124V/80 ohms = 1.55A. So the forklift battery charges a little slower, but our charger still works.

Steve
 
Re: battery charger

Q1. If you have 48 volt (8) 6volt batteries in series, do you have to have a 48 volt charger to charge the batteries or will a 12v charger put some charge on the batteries but not as fast?

A1. You would need a 48 volt charger if the batteries were left in series to form a 48 volt bank. Or you could put two of the 6-volt batteries in series and charge with a 12-volt charger.


Q2. If you parallel (2) 12v batteries will a 12v battery charger still are sufficient to charge the two at the same time or would you have to disconnect them and charge them one at a time?

A2. Sure you can, it will take twice as long to get the batteries charged. The time it takes is completely dependant on the size, age, temperature of the batteries and the size of the charger. Flooded batteries are best charged at a rate of Capacity/8-to-24. For example if you had a 1000 AH batteries, your charger should be rated at 125-to-40 amps. So in this example you could put 3 1000 AH batteries in parallel, and with a 125-amp charger, you could recharge them in the float mode in 24-hours


Q3. If you have (2) 12v batteries in series for a 24v system do you need a 24v battery charger to do any good?

A1. Same as Q1. You would need a 24-volt charger.
 
Re: battery charger

I'm taking it that 2 batteries with a different amount of charge when connected together in parallel will result in the higher charged battery lowering it's voltage (or trying to charge the 2nd battery) until equalized with the raising of the voltage on the lower charged battery? thus ending up with a voltage somewhat less than the desired 12 volts but more than the voltage of the lower charged battery originally?
 
Re: battery charger

Not sure I understand you correctly.
If you were to take a fully charged string and batteries and put them in parallel with a discharged string of batteries without a rectifier/charger and let them equalized, you would end up with two strings of equally undercharged batteries.

Edited for clarity.

[ October 06, 2004, 03:52 PM: Message edited by: dereckbc ]
 
Re: battery charger

petersonna, parallel battery strings are common practice. It is done for added capacity to battery operated equipment. The only trick, if that is what you want to call it, is the battery should be of the same type of technology with the same float voltages. For example; flooded batteries typically float at 2.16 VDC per cell, while VRLA's float at 2.25 VDC per cell

I have hundreds of offices that use 10,000-amp @ -48 VDC plants with 14 strings of 4000 AH batteries. This gives the plant 4-hours of reserve capacity, giving the generators plenty of time to come on line.
 
Re: battery charger

I believe most batteries are rarely (if ever) placed directly in parallel.
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My F-250 diesel uses two 12v batteries wired in parallel. No diodes. The problem is that if one battery shorts out, it will take them both out.

My farm tractor also uses two 12v batteries wired directly in parallel. In fact, I see this quite often with farm equipment unless they use 24v for starting, in which case the batteries are wired in series.

Bob
 
Re: battery charger

My mistake about the batteries in parallel. But one should be cautious when placing them in parallel, as dereckbc pointed out. I would guess that you would want to charge them in parallel, so they both have the same charge and voltage.

Steve
 
Re: battery charger

One of the problems with batteries in paralel is internal resistance. I don't know what it is with lead acid batteries but NiCads are around 1 ohm give or take an ohm. So they dont only charge each other they can also heat each other up and hot batteries in paralel probably wont reach any kind of equalibrium until they're down to a couple microvolts.

Charging batteries is mostly a matter of the charging voltage being a little higher than the battery voltage. Then to increase the charging current you increase the charging voltage.

So two 12 volt batteries in series wont take a charge from a 12 volt charger unless the sum of their voltages fell to less than 12 volts. And then it would charge them to 12 volts but not 24.
 
Re: battery charger

In my work van I have 6 deep cycle 27F 12 volt 280AH Gel-Cell battery's that are mounted in a ventilated cabinet right behind my drivers seat. they power a 240/120v 3500 VA Triplet full sine wave output inverter that powers receptacles at the back and side doors of the van, and a 110v 700 VA Victor square wave inverter into a Triplet line conditioner (true 120vac sine output) that powers some receptacles that I use for some lighting and chargers for my cordless tools in the van. (Beats dragging a generator around) the batteries are recharged via a 280 amp alt. that I replaced the original with. the charging current to the inverter battery's is controlled by a automatic battery maintainer that prevents over charging. and a bulldog auto start system that will start the van motor to keep the battery's from going below 11.5 volts. I haven't had to replace any of the battery's yet, and I have not had any problem's with running all 6 in parallel. I should state that there is a set of free-wheeling diodes that keep's the truck battery separate from the inverter batteries.

There's no way I'll go back to dragging around a heavy generator ever again as with the 3600 VA I get more power out of it than I ever got out of the same sized generator.
 
Re: battery charger

Originally posted by dereckbc:
petersonna, parallel battery strings are common practice. It is done for added capacity to battery operated equipment. The only trick, if that is what you want to call it, is the battery should be of the same type of technology with the same float voltages.
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I agree you can do this but my point was more along the line of you can't just do this without taking appropriate measures to be sure the things will work.

For instance, you would typically not want to take a new battery and one that is used and put them in parallel. Or use two different styles of batteries.

I probably should have elaborated.
 
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