Bulk charging - bypassing the 3 stage charging, should I do it?

[GaryJ]

I came into possession of 12 Enersys DataSafe 12v AGM batteries from a 3 phase ups. The manual states a float charge of 2.265 VPC at 77°F, no problem there. However the fast bulk charge they recommend occasionally (4 or 5 times a year) the battery may be recharged at 2.40 VPC 

So should I keep the bulk charge setting the same as the float setting and follow the guideline of occasionally charging at 2.40 VPC? This would bypass the 3 stage charging on my C40 controller for most of my charging.

http://www.batterystore.com/content/Catalogs/Enersys%20DataSafe%20HX%2012V%20Front%20Terminal%20Batteries%20Installation%20%26%20Maintenance%20Manual.pdf

Any help would be much appreciated and thanks in advance!
Gary

[dnix71]

The 14.4v charge (2.4v/cell) is intended to equalize the stack and prevent hard sulfation.

Too high a charge causes positive grid corrosion and gassing, but if you don't get the battery voltage up there occasionally you will have problems with an unequalised battery (overall capacity) and sulfation. Whoever made the battery should be assumed to know what they are talking about.

[DanG]

Unsaid in the linked document is an explanation of 'Fast Charge' and when it is to be invoked...

'Occasionally (4 or 5 times a year) the battery may be recharged at 2.40 VPC with a current limited to the values listed in the Table 1. This charging should not be allowed to continue for more than 16 hours'

During commercial back-up service you might have grid/generator/source outage events occurring with short notice that might require deeply drawing down battery string charge, the 'Fast Charge' is to prepare for the next outage(s)...

When faced with the two scenarios, high-rate charging or over-discharging a battery string, both (theoretically) reduce the strings' projected lifetime, the lesser of those two evils is 'Fast Charge'.

It's meant for when it's in your better interests to cook the string a little to prepare them for the next deep-draw down.

Example - Service call responding to a trouble code from a remote installation with grid failed & propane back-up generator faulting out  - you show up with a temporary generator and those expen$ive  batteries are already low low SOC, your mission is to get batteries back to 97% ASAP since you're pretty sure the grid will remain flaky and the sites' automatic generator needs an overhaul; fast charge the batteries and come back the next day and swap out the automatic generator would maybe better ensure the least damage occurs...

[mab]

it kind of depends how hard you're working them - obviously in a UPS they don't often go off float so don't need equalization at 2.4vpc.

you can equalise a battery without going over float voltage, but it takes continuous charging at float for 24-48 hrs typically (maybe a bit longer if they're a long way off equalised), which is difficult to achieve if you're using them every day. If you can do that (if you have a continuous source like hydro for instance, or if you're not using the batteries every day) then you may not need to go to 2.4vpc.

If you're using them lightly every evening and recharging fully with solar every day then you may have to equalise every month or so; if you discharge them more deeply and they don't always recharge by the end of every day, you may have to equalise every week.

The problem with charging at 2.4vpc is you are going to accelerate the loss of water from the electrolyte; If the batteries are fairly 'new' they probably won't gas much, but as they get older they seem to gas more and lose more water - this isn't a problem though as you can prise off the plastic covers then pop the caps off and carefully add deionised water to an AGM (being careful not to overfill) - I've had to do this with mine.

[GaryJ]

Thank you all for great feedback! Yes I plan to use this bank lightly and only deep cycle backup during outages. I should be able to adapt to the battery maker charging specifications. The float and bulk charging will be the same at 53.6v in my 48v setup. Every month or so I will hit the equalizer button on the C40 controller which will hold voltage 4 volts above the bulk setting, 57.6 (2.4 VPC). I may have to push the button several times to get the bank up to the level but much better than having to manually adjust the potentiometers each time.