Charging AGM batteries - Nerdy battery charger question

Our team has four CTEK MXS 5.0 battery chargers. Up till now we have used them in the conventional mode. Just recently I have noticed the CTEK charger has a AGM mode. Now I didn’t know what this was until I looked it up. I have always assumed the Greenpower batteries are a gel electrolytic and AGM was for some funky new car battery technology. AGM stands for Absorbent Glass Matt and it turns out the Yuasa Greenpower battery has this technology inside it. Still a bit sceptical so I turned to the datasheet of the battery. This says that for cyclic or boost charge the voltage should be 14.52 V (+/- 3%).

photo Yuasa REC36 battery_zps8vzbuycj.png

Now the CTEK charger handbook as a handy diagram showing the charge states. See below:

photo Ctex mxs 5_zpsdwlvo2tu.png

This shows that in AGM mode the charge voltage is 14.7 V and in normal mode 14.4 V. So it looks like AGM mode is within the 3% margin of the battery so shouldn’t cause it damage but the normal charging mode appears closer to the 14.52V the battery prefers.

So my question is what is the point of this AGM mode and will it make a difference to either increasing the energy stored in the battery or reducing the battery's lifespan.

Interestingly for float mode the charge voltages are identical in either mode.

See link for more details about AGM.


  • edited October 2016
    That's a tricky one, I don't think it will make a great deal of difference though. The datasheet's are not that accurate and can be misleading. We are just starting to use the new REC36-12 batteries having mainly used the old YPC33-12 batteries. With the YPC33-12 they have a relatively low terminal voltage when fully charged and high current flows if you charge greater than this. If you charge them with 14.4 volts the current flow is still quite high even when fully charged, This causes issues with chargers as the current doesn't fall low enough and they sometimes don't switch off damaging the batteries as they continue gassing reducing the fluid level (especially if paired up). We use CTEK XS 7000 chargers, mainly, charging two batteries in parallel.
    We are not experts, however our brief tests with the REC36-12 seems to indicate that when fully charged terminal voltage is higher and the current flow drops off quickly when charging even above this. We have charged them at 15 Volts and more and the current still drops off. This seems to indicate that charging at a higher voltage is ok and will probably help balance the charge in the cells after a typical GP deep discharge. Worth keep a eye (actually an ear) on them at the time they would be just getting to fully charged (amount of gassing). Also we think it is normally worth leaving them on float charge 13.6V for some time after as this helps put a little more into them.
    Would be best to do some experiments using a battery discharge rig (11 Volts min) if you can afford a spare battery to do the tests (every deep discharge will damage them).
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