Thread: Charging a six volt battery.

Probably not a good idea.
Do you have two 6V batteries ? - you could put them in series and charge both of them together.

It all depends on the battery you are talking about and what it is in.

As a general rule, if you connect a twelve volt supply to a six volt system, as with jumper leads, one of three things will happen -
a) the voltage in the six volt system will rise to close to 12v

b) the voltage in the twelve volt system will drop to not much more than 6v

c) there will be a voltage drop of around 6v in the connection.

a) will almost certainly destroy parts of the system - ignition modules, bulbs, electronics, probably the battery. This is what will happen if you use jumper leads form a larger 12v battery.

b) If you are using a low current charger, the charger voltage will drop to not much more than 6v. The current draw is likely to cause the charger to overheat, trip a thermal overload switch, or blow a fuse.

c) This is really where you want to be. If you charge through a suitable bulb, this will allow you to use a 12v charger. To calculate the size of bulb, have a look at the maximum continuous current that your charger will deliver. Multiply this by six to give the wattage you want to dissipate. For example, if the charging current is 5A, then you need 30W. While you could use a 30W 6V bulb, if the 6V battery is flat, then the initial current may be enough to blow the bulb, so you are better to use a 60W 12 bulb, which will not blow, and will limit the current to a safe level, while still allowing around 5A when the battery has reached 7V (I have ignored changes in resistance of the bulb with temperature, but the accuracy required is not great enough to worry about this).

John

Originally Posted by waynep

Probably not a good idea.
Do you have two 6V batteries ? - you could put them in series and charge both of them together.

Years ago, in the 1970's I had a six volt VW that I converted to twelve volts. We used a whopping big resistor to drop the voltage to the wiper motor, perhaps something as a resistor from the twelve volt charger may work, do not know for sure. The starter motors on the VW could take the twelve volt so the starter motor was not a problem. Just needed a new genny and generator stand from a later 12 volt model, change light bulbs, radio, and the whopping big resistor to the wiper motor were the main things we needed, the wiper burning out if it did not have the resistor. Think we carried a 12 volt coil incase the 6 volt coil burnt out, but do not remember now. So perhaps your problem is not insurmountable. Talk to experts to see if a big resistor will drop voltage for you, and can be used in the charging circuit. Did once have a duel 6 and 12 volt charger. Perhaps you can get one of these second hand, should be cheap if you find one. If mine is still in the shed it has not worked for many years. talk to owners of old VW etc, might still be some around. some other cars were also six volt.
Did caving once, and had a rechargable lead acid miners light--still have it without battery, the old lead acid battery case being made to hold dry cells. That was a 2 cell lead acid battery. We charged it from a car battery using just a couple of large resistors to drop the voltage on a commercially built battery charger. So I am sure you could rig up a system with just a couple of resistors to charge your 6 volt battery from your 12 volt one. In junk shops are lots of chrgers for old laptops. etc. Some of these are 7.2 volt. I would think with a cople of clips to jion to your 6 volt battery one of these could be used as a tricle charger. You do put in slightly more voltage than the battery rating to charge it. Small lead acid batteries are available for many uses, e.g. professional photographers flash guns like the Metz 60 CT2, etc., which are now out of date. Picked one up in a clean up day and seen others in junk shops. a charger from that would maybe work. You need to talk to people with more knowledge than me.

Generally not a good idea, as the charging current will be too high.

Depending on how desperate you are, you could still use the charger, but you need something to reduce the voltage being applied to the battery so you don't cook it.
Years ago we used to use an old wire element from a electric heater as a big resistor when charging motorbike batteries.
You connect the element in series with the battery and charger, and the resistance of the element will drop the voltage being applied to the battery.

We'd connect one end of the heater element to the battery and then try clipping the lead from the charger onto the element in different places until we got the right charging current.
The wire element from an old toaster would probably do the same job, or the ballast resistor from a car with a 6v coil (depending on what you have access to).
You probably want to end up with around 7-8 volts if you measure across the battery terminals while charging.

hth

I bought a cheap 12v/6v charger from repco for $38 - why not do that?

Garry

Back in 1973 I had a 1948 straight 8 Buick it was a 6 volt system, belive or not, 8 cylinder starting off 6volt. While I was restoring it in stead of buying a new 6 volt ( having one made ) battery I us to jump start it of a 12 volt It never did an harm .

Hodgo

...

Gents and Ladies,

A battery should not be treated as a resistor but as a capacitor (with a small series resistor or a large parallel resistor) . Voltage division across resistors as per a heating element, ballast resistor or light bulb is fine initialy when the current is limited by the internal resistance of the charger. Eventually the current will drop off an so will the voltage drop across your resistor/bulb/heating element and your battery will be overcharged.

You must monitor the battery to ensure that you do not overcharge it or set the charger up on a time operated switch to turn off after a period where the amphours of charge have been achieved. Based on a C/5 or C/10 charge rate where C is the capacity in amphours and the result is given in amps from totally flat it would take 5 or 10 hours to charge the battery depending on what rate you selected. A few other fudge factors involved like you should never totally flatten your battery if you want it to last more than a few cycles of charge/discharge so what percentage of charge are you really putting back in to get it to full charge (which for most auto electrical purposes is taken as somewhere between 80% and 90% charged) Also what is the efficiency of the battery (there is a more techy term which escapes me at the moment) some batteries require 150% of charge to be put back into them to to get then to full charge).

In summary monitor your battery unless using a charger that will drop back to a trickle charge once the battery is getting close to charged.