Mitsubishi 4WD Club Trev's Tip 17. 

Trev’s Tips

Charged

This months Trev’s tips is one that would affect any club members that run 12 volt systems through their caravans, campers or boats that have a separate power source from their cars.  A number of weeks ago I came to the sad realisation that in our household we have a total of 7 12 volt batteries to keep motor vehicles etc running and will soon be adding one more to this collection to go on a camper trailer (more on that latter).  All this power raises questions about how to keep it all charged up.  As we all know common lead acid batteries don’t like being discharged so it is important to keep them in a fully charged state.  Naturally the 2 batteries in my car, one in Rachel’s and one in the motor bike have no problems having their power replenished every time they are taken out for a drive.  The batteries that I’m really concerned with are the ones that receive little use and maintenance.  In particular I have two batteries in the boat and one smaller spare that I use for crabbing and some lighting purposes when fishing and camping.

In recent times I have had to replace both the boat batteries which I am convinced went flat due to lack of regular use (they were only just over a year old).  Although I managed to charge them up they would not hold charge and for safety reasons were considered unreliable in the boat so were replaced.

This got me thinking about how I could keep these batteries charged while not in use.  The obvious answer was to get a solar panel that would provide a small trickle charge to the batteries each day and could also be used whilst out on camping trips.  After much investigation I gave this idea away, partly because I had a better one but also because it seemed that to keep three batteries charged I’d really need three solar panels (according to the expert salesman) which was going to be very expensive.

After speaking to a number of people about this, it was then suggested that perhaps I could use the 12 volt charger from the cordless drill that I almost threw out a few months ago to charge the unused batteries.  I was a bit sceptical about this but after some testing it appears to be working very well. 

From there it was just a matter of plugging the charger into a power socket and connecting the alligator clips to the battery.  There you have it, a home made charger.

Since this time a have made two more charges in the same manner so my two boat batteries and the spare are all being regularly charged.

There were a number of things I had to consider before doing this which I have listed below.

1)             Firstly you need to check that the charger’s power rating is correct for the intended purposes.  Somewhere on the charger it will give you the DC output of the charger.  You should be looking for a 12 volt DC output charger.  However after saying this I did find some unusual outputs from each of the charges when I actually checked them with the multimeter.  More on that shortly.

2)             Secondly take note of the output amperage.  Ideally to trickle charge a battery an output of 450ma is good.  This is about the same output as a 5 watt solar panel.  However one of the charges I am current using is rated to 1500ma (1.5amps).  It should be noted that this current output will only occur when the battery is flat.  If the battery is well charged then charger will be putting out a lot less current than this. See the table below.

Here are some interesting Stats (for those people that are into this kind of thing) that I discovered when testing my three chargers with a multimeter.

Let me try to explain what this all means with my limited knowledge.

The claimed output column is what I read straight off the actual charger. The charge in the battery is self explanatory. The actual output of charger are the readings I took from the charger by using a multimeter.  Notice anything unusual? 

Let’s look at the results of the first charger.  Its claimed output was 9 volts.  This means it should really only be suitable for charging 9 volt batteries.  However after testing it I discovered it was actually pumping out a staggering 21.7 volts.  The amperage reading is obviously lower than the specified because the battery was already at 13.2volts.  Remember I said earlier that a charger would only put out its maximum amperage when the battery was flat.  Once the battery has taken some charge this amperage will drop and will eventually become 0ma when the charge in the battery matches the output voltage of the charger.

This can be seen perfectly in the results of the second charger.  It was pumping out 12.25 volts according to the multimeter and because the battery was already at this level of charge the amperage dropped to 0ma meaning it was not giving the battery any more charge, just maintaining its level.  Perfect.

The third charger was similar to the first in that it was putting out a lot more voltage than was claimed and even though the battery was already at 13.9 volts, the charger was still adding more charge to it as shown by the 460ma reading. It should be noted here that these chargers, unlike a properly designed commercially built model, will not switch themselves off once the batteries have  reached full 12 volt charge unless their output voltage is at a correct level such as charger 2.  This meant that the results of the first and third charger were of some concern to me because they would continue to charge the batteries till they reached 21.7volts and 19.4volts respectively. Obviously long before this would happen the battery would boil dry and become ruined.  To get around this problem I connected all the chargers up to a power board which was in turn connected to a timer.  I then set the timer so that it only turns on for 1 hour per day. This just ensures that the chargers aren’t left continually running to overcharge the batteries.

This is a cheap way to ensure that all the batteries laying around the place for your other recreational hobbies are well maintained.  It also gets rid of some of those old chargers laying around clogging up your shed.  Who’d ever thought that the $15 cordless drill I wrote about a few weeks ago would have so many useful parts.  After nearly throwing it in the bin when the battery went flat the drill itself is now being put to good use and now, so to is the charger for it.  Problem solved.

One last point for this month.  I noticed that in the last specials catalogue from Super Cheap that they were advertising a proper trickle charger that would charge your battery to its full capacity then go into monitor mode whereby it would not apply any more charge till it was required.  It was only about $15, so if you have only one battery that needs to be maintained in this way then perhaps it may be easier to just go and buy something like this.  However if you have a lot of extra batteries like me or are just up for the challenge then this is a very interesting project for you and will help you understand a bit more about 12volt systems. 

Nowadays batteries are too expensive to replace on a regular basis so do yourself a favour and start to look after yours properly now through a system such as this and get the full benefit of extended battery life as well as the piece of mind of knowing that your battery should be in tip top shape next time you want to race out for an unplanned weekend trip somewhere.

PS.  Whilst I am quietly confident that this setup should work fine, I am by no means an expert, so if you are more knowledgeable than me on 12 volt systems and know of a reason why this is a bad idea I’d be keen to hear your advice.

Till next time

Happy 4wding

Trevor

Help me to be the person my dog thinks I am.