How to charge Sealed Lead Acid 'Gel' Batteries

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Now I may look like an idiot in a Stormtrooper uniform but as well as working as an extra on "Star Wars IV" - yes I actually appeared in the film - I also have a Ph.D. in electronics, so let's see if I can help you with your battery problems.

This about sealed lead acid batteries, otherwise known as Gel batteries, are used in many household items such as ride on toys, golf trolleys, invalid carriages, electric wheelchairs, computer back-up power supplies,rechargeable high power torches etc. etc. There are several other types of battery around in particular Nickel-Metal Hydride (NiMH) and Lithium. You simply MUST use the correct supplied charger for these as they can and do catch fire if mistreated so do not use this guide when charging them, also don't be surprised if airlines prohibit them being flown as they can catch fire. All sealed lead-acid batteries can however be used in any orientation, even upside down as they can't spill in normal use.
If you buy a sealed gel battery - commonly made in the UK by Yuasa - to replace an identical battery that has failed then there is no problem, just connect and carry on using as before. What you must NEVER do however is confuse these Gel batteries with car batteries - the type that you put water inside, and you must certainly never ever use a car battery charger on a sealed battery. It will blow it up before you can even think guarantee claim!
So how do you recharge Sealed Lead Acid batteries, and what are the most important points to remember?
First if you have a supplied charger for whatever item the battery is powering - use it before any other type of charger, but check first that it is working properly. If your charger is faulty it may easily be the reason why the battery needed replacement in the first place! Faulty chargers can ruin rechargeable batteries before you have time to blink. To check that the charger is working properly use a test meter to measure current and voltage. 

If the original charger is not to hand then take a look for a charger that is specifically rated for sealed lead acid batteries - look on eBay. If you have to use a non specific charger then you first really need to go out and buy a cheap multimeter to check how the charge is going. These can be bought on eBay for well under a tenner.
To measure the battery voltage first switch on the meter and set it to a suitable voltage range - usually the 20Volt range.  Most meters have three sockets for the leads to be plugged into. The black negative lead usually stays in one socket all the time and then the red positive lead can be plugged into a choice of two sockets. One socket is for voltage V readings and the other socket is for current A tests. If you plug into the current A socket and then try to measure the voltage you stand a really good chance of damaging the meter, AND possibly of damaging your battery too!

To actually read the voltage first disconnect the battery from any attached equipment and then put the meter leads onto the battery terminals - red to red, black to black. Then simply read off the voltage shown. The normal point at which charging a nominal 12Volt battery should stop is usually when the meter reads 12.87 volts. Yes, you can easily “squeeze a bit more juice in” but this may well reduce the life expectancy of your battery. There should be no problem leaving a correctly set up meter connected to read the voltage while your battery charges.
The current range on your meter is to measure how much electricity is going into the battery AS it is being charged. Unfortunately when measuring current the circuitry inside the meter will overheat if left connected for more than a few seconds, so you'll have to keep measuring and then removing the meter - inconvenient but necessary!

To measure the current going into or out of your battery set the meter to a suitable A range - usually 10A - and connect the leads IN LINE with the battery or batteries. So one lead goes to one of the battery terminals and the other goes to the charger or device that is being powered. Instead of this you could connect it BETWEEN two batteries connected inline if that is easier - what goes into one battery will automatically go into all the other batteries connected in series.

So to summarise, you connect one meter lead to one of the wires from the charger and the other meter lead to the battery itself to read the current. In the olden days you had to be extremely careful which way round all this was connected. Fortunately things are much easier these days! If you connect the meter the wrong way round you will just see a little – sign before the numbers but the reading is perfectly valid.

To reiterate the meter is connected BETWEEN the battery and charger to check the charging current, and the meter is connected ACROSS the battery to measure voltage.

There is an excellent free guide to the use of different sorts of Gel batteries on the Yuasa web site called The Little Red Book of Batteries. We earnestly suggest that you download it and then read it!

By the way we recently saw a chap being denied permission to take a sound editing unit powered by a rechargable Gel battery as hold baggage in a commercial aircraft. Until recently we would have seriously critiscised any airline for denying permission to carry a rechargable battery in an aircraft but there have been several fires caused by people not identifying this batteries properly as some newer types can be dangerous and catch fire. Why should the average user know the type anyway?  Most sealed lead acid batteries are completely safe to take on aeroplanes. Why? They are actually used to power the avionics on commercial jets when there is no ground power supply, the plane probably had three 60Amp sizes of these batteries installed already! They are only used for one year and then thrown away. Even gliders have a 17Ah sealed battery inside to power their radio! The ground staff obviously didn't know the difference between a car battery with its liquid acid and a sealed Gel battery, after reading this guide you will.

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Thanks, Rob.


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