AGM OR GEL Batteries - POWERCHAIRS or SCOOTERS

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DIFFERENCE BETWEEN GEL & AGM BATTERIES AS USED IN POWERCHAIRS & SCOOTERS

And the the CHARGING differences.

Powerchair stuff ONLY here

In our power wheelchairs, or scooters, there are only two types of battery in common use in this century. We SHOULD be using lithium in 2013 for at least 6 years or so, but sadly we are not. (however I am!) However I digress!

These two types of common battery are Gel batteries & AGM batteries. Both are (or should be) high quality and deep cycle capable. Both are SEALED or rather valve regulated which is the same thing as far as we are concerned and so don't spill any acid.

Contrary to common belief, both still use acid. but it's immobilised internally. Either held in a GEL state (via added silica gel crystals to make it into a very thick paste) or soaked into a thin glass fibre mat or tissue in between the cells lead plates. So neither type can spill any free acid. This is the physical difference between AGM and gel batteries. They don't leak, spill, and are maintenance free.

Lets get a few important things out of the way first.

1. You cannot afford cheap deep cycle batteries. The best lead type batteries available on the planet are not good enough in a powerchair. They are the weakest point of your scooter or powerchair by far. There are in fact just a small handful of suitable batteries on the market that are even worth considering. And the best one to choose depends on your priorities. Both have different shortcomings.

2. GEL batteries such as the High Quality MK brand, or the identical technology Sonnenschein brand are the ONLY gel batteries worth buying. Anything else is a waste of your money and will either perform poorly or last a very short time. Or both.

These are in fact, the only batteries that 90 percent of all powerchair or scooter users SHOULD be buying. IF GEL batteries are charged correctly, and most are not, then these batteries offer the best cycle life of all lead based batteries available. If this is your highest priority.

Around 500, 80% deep discharge cycles, can be expected. That IF above however, is very important, as we will see later on. And these two gel batteries also offer an adequate performance for most normal users in average powerchairs. If this is you, buy these only.

3. Normal AGM batteries are a little bit cheaper, offer no benefits over gel batteries. have a cycle life of around 300 to 400 cycles @ 80%. Some MUCH LESS. So I wouldn't really recommend these over the GEL batteries above. The saving isn't worth it and both performance and longevity really can suffer. So forget these. No matter how good the deal seems!

4. However there's another type of AGM battery with pure lead plates that costs a little bit more than both of the above batteries. These are batteries such as the Optima (less suited for tech reasons to powerchair use), Enersys Odyssey (inc rebranded versions such as Die Hard, Stingers, and such), plus one or two other names.

These batteries have one big advantage over GEL or normal AGMs. That is they have a markedly lower internal resistance, and also much lower Peukert affect. That means that under heavy loads, or fast discharge conditions they markedly out perform the best normal AGM or the best quality GEL batteries. More of their power is ACCESSIBLE to us fast. These are very popular for starting trucks in Siberia for the same reason or in very hard to start race cars. Or in powerchair soccer where big amps are called for continually, or for off road use as the voltage drop under load, is much lower and torque is improved.

This is especially so as the batteries become more discharged in use. The can still perform well compared to a gel battery when very discharged. These are the batteries I personally prefer in my own powerchairs. You will likely get 400 cycles however rather than 500 from the gel batteries. So you are trading some cycle life for hi-Amp capability. This matters greatly to me, because I find Gels inadequate for my own powerchairs. Because of the lower Peukert effect of these pure lead batteries a lighter smaller Ah battery also offers similar range to a slightly larger heavier gel battery and charges much faster too.

CHARGING

Almost ANY charger will work on any lead based battery to a large degree.

However, if you want to get the best service life from your investment then read the following carefully. There isn't a single charger on the planet that can correctly AUTOMATICALLY charge both AGM and GEL batteries and know which is which. The charger does NOT know which battery is on the end of the cable. Contrary to what you may have read on other less well informed sites.

So if you don't want to drastically shorten the life of your expensive batteries you MUST choose the correct charger for each battery type.

This isn't very easy, as most manufacturers do not say, or give full detailed charge algorithm details or voltages or other specs! They just claim that they can charge all batteries. This is true, but not properly! So you need to find out one way or another, or do as I do and simply use a charger that is totally USER adjustable in all of its settings.

The "automatic", "charge all", AGM/GEL chargers simply do neither correctly. Most simply charge at 14.4v (28.8 for two batteries) and hope all works out OK for all types. Well they do charge both in a fashion. But they seriously shorten the service life of the Gel battery, possibly undercharge the Pure Lead batteries like Optima, Exide pure lead, Odyssey, Di-hard rebrands etc. So they are not very clever. So they work, but badly.

CORRECT CHARGE

MK states for their GEL batteries at 20C / 65F room temperature to charge using the following parameters:

CC (Constant Current - stage 1) This is the max power of the charger, we are limited to around 8 to 12 Amps due to the charging connector on most powerchairs. Otherwise around 20 (1/3rd its capacity) is a better figure. This isn't critical it just takes longer to 14.10v MAXIMUM. Charging to higher than 14.10V per battery shortens service life quite seriously. Even a few tenths of a volt is a problem here!

CV (Constant Voltage - stage 2) This is where Voltage is held at 14.10 Volts until the current (Amps) naturally tails off to an extremely low level. This should be well under .7 of an amp, preferably much less. (1/100C to 1/500C is suggested by MK or 8 hours).

FLOAT (Float is a final indefinite stage) Voltage is held around 13.5v to finalise charge over time and to prevent self discharge if unused giving long term protection.

MKs own tech spec for charging their Gel correctly is here

A gel battery typically takes double the time to charge compared to say the Odyssey I use. The Odyssey charges much faster because of 2 things. Less surface charge effect & a greater difference between charger voltage and battery terminal voltage.

ODYSSEY AGM (normal AGM) states for their batteries at 20C room temperature to charge using the following parameters:

CC (Constant Current - stage 1) This is the max power of the charger, we are limited to around 8 to 12 Amps due to the charging connector on most powerchairs. NO AMP LIMIT needed if connected directly to 14.70v (14.4V) MAXIMUM. Charging to higher than 14.7V (14.4V AGM) per battery shortens service life.

CV (Constant Voltage - stage 2) This is where Voltage is held at 14.70 (14.4) Volts until the current (Amps) naturally tails off to an extremely low level. This should be well under .7 of an amp, preferably much less. (1000thC or 0.07 Amp is suggested by Odyssey for best cycle life). Cutting charge short, also shortens service life.

FLOAT (Float is a final indefinite stage) Voltage is held around 13.6v to finalise charge over time and to prevent self discharge if unused giving long term protection.

Odyssey own tech specs inc charging is here

If you want - just ignore the above, its easier to say, hold it at 14.7v until it no longer pulls enough amps to measure... (1000C) Done!

As you can see here, its impossible for one charger to charge at both 14.10V (Gel) and 14.4V (AGM) or 14.7V (Odyssey/Optima/Stinger/Di-Hard rebrands etc.)

So it is essential to use a correct charger for your specific battery or drastically shorten the service life of that battery. Not sure what yours does? Get a GOOD digital volt meter and monitor the voltage towards the end of charge.

RED SECTION STAGE 1 (CC or BULK) SHOWS CHARGER AT FULL POWER AND BATTERY VOLTS CLIMBING. If you had a powerful enough charger this "stage" wouldn't exist, and it would just jump up to the CV stage instantly.

ORANGE SECTION SHOWS STAGE 2 (CV or ABSORPTION) WHERE THE VOLTAGE IS HELD STEADY AT EXACTLY 14.1V (GEL), 14.4V (AGM), OR 14.7V (PURE LEAD LIKE ODYSSEY) AS SOON AS THIS STAGE ENDS, YOUR CHARGER SAYS DONE!!! ITS NOT... ITS ABOUT 98 PERCENT DONE.

GREEN SECTION (STAGE 3 FLOAT) CONTINUES AT A LOWER VOLTAGE INDEFINITELY. IT FINALISES THE CHARGE TO 100 PERCENT NEEDED TO PREVENT BATTERY KILLING SULPHATION, THEN PROTECTS AGAINST SELF DISCHARGE & CAN BE LEFT CONNECTED FOREVER. 13.5V TO 13.8V IN CYCLIC USE.

BLUE SECTION, NOT MANY CHARGERS DO THIS. YOU CAN IGNORE THIS. THEY USUALLY CONTINUE WITH THE GREEN PART INDEFINITELY AT 13.5V. IT IS A JUST A FURTHER DROP IN VOLTAGE FOR VERY LONG TERM (MONTHS +) OF STORAGE, AT 13.2V AND NOT REALLY IMPORTANT IN A POWERCHAIR DAILY CYCLIC CHARGER.

SUMMARY

PURE LEAD AGM E.G. ODYSSEY (rebranded as Di-Hard etc, stinger and all the rest Optima, Exide Pure lead spiral etc) HI AMP capability, needed if your chair is programmed to go, want some some power on ramps, off road, sport use, fast charging, etc

GEL best cycle life of all provided it is CORRECTLY charged, not at 14.4v (28.8v) as most mobility/universal chargers do. Less good for powerful chairs, steep hills, sport, off road. You FEEL the voltage sag as you try to get the power in these situations. They get much worse as they become discharged. The internal resistance increases as the battery charge becomes used up. In a slow chair, or a chair with delays programmed in, as most are, you would never notice this.

Burgerman.

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