Charging batteries and battery voltage?

[EricMark]

OK I was an auto electrician back in the 70's and 80's so under stand a lot about batteries, however at that time the sealed for life battery was rather new, and you may have had a VRLA in a fire alarm, but not in a vehicle, the AGM battery was unheard of.

So did have stage battery chargers for fork lifts and like, but for the home battery charger it was a poor quality transformer, rectifier, and built in ammeter, would start charging at 8 amp and when it had dropped to 2 amp considered charged, we did have fixed voltage 13.4 volt if stand-by, and 13.8 volt on the car, some could go to 16 volt, that was the open circuit voltage for a RB106 or RB108 regulator with a dynamo, but would rarely reach that voltage with battery connected.

But today the 8 to 12 amp home battery charger seems to have been replaced with a 3.8 amp "Smart" charger, the one I have three user settings under 12Ah and over 12Ah and cold conditions. It has 3.8, 3, 0.8, 0.1 and zero amps it charges at.

So it will drop current in stages and 3.8 and 3 amp are not a worry, but the 0.8 and 0.1 amp range can it seems raise the battery volts quite high, up to 14.4 volts, and with a good battery what it does is raise to 14.4 volts then drop to 0.1 or zero amps depending on range, so left on charge the battery normally shows 12.9 volt as it drops to 12.8 charger cuts in, volts go to 14.4 and then it switches off again and it is rare to see the volt meter show anything but 12.8 volts.

However I get the odd battery that does not raise to 14.4 volts within 24 hours. I have a 75 Ah battery on charge at moment, at 0.1 amp the voltage is slowly raising, yesterday 13.4 volt today 13.8 volt at which I am not worried, but as it getting higher, I wonder should I take battery off charge, or wait until it starts to cycle? Can't see it doing any harm, but it seems this battery is between the stages, never reaches the 14.4 volt needed to switch off, so sits at a high voltage for weeks.

What I have as well as three cars is batteries which sit there for months maybe years doing nothing, and the idea is to alternate the charger between batteries two weeks on each to ensure fully charged, should I stop, and if so how often should I charge.

I have a 7 Ah used to power radio, 12 Ah in a jump start case, 2 x 35 Ah to run moblity scooter, 75 Ah from caravan (Only one not VRLA/AGM) and a 90 Ah out of Jag used as back-up for caravan, plus in the cars a 40 Ah sealed but not AGM, a 90 Ah AGM and a 120 Ah flooded. And two Lidi 3.8 amp smart chargers. Plus some old chargers never now used.

 

[Lucien Nunes]

The behaviour you describe, where the charger gets stuck in bulk charge or equalise mode and never either starts a timed constant-voltage equalisation or trips back to float is quite common, due to either a battery that is too large for the charger, load connected to the battery during charging or incorrect temperature sensing. I would not generally use a 4A multi-stage charger on a 75Ah battery and I think many chargers of that size would not recommend it. A unit that could easily complete the charge in 12-18 hours, perhaps 8-10 amps output for this size of battery, is much more likely to follow a predictable voltage/time curve and tolerate variations in battery condition, temperature etc.

'Smart' multi-stage chargers are not all created equal! A straight IUoU 3-stage will never terminate the bulk charge if the terminal voltage is low, or never terminate the equalisation if the current is too high. Good chargers have watchdog timers for each stage that will abort if the battery does not respond in an expected way, that may be adaptive according to the charging progress. For example, a charger I used to specify would record the bulk charge time and then calculate a maximum equalisation period that depended on both the bulk charge time and change in temperature. Once that equalisation time limit had elapsed the charger would drop back to float regardless of the current. Therefore it was impossible to get it stuck in equalisation mode and start venting the battery due to standing load from a connected system.

 

[EricMark]

I have used a charger like you describe with a narrow boat, but it was designed for a flooded battery, where I could top up water level, 35 amp is a massive charge rate and OK with 3 x 140 Ah batteries for domestic and 1 x 140 Ah for engine, but the charger I have is designed for VRLA or AGM two names for same thing. And has two outputs zero to 12 Ah and 12 Ah to 120 Ah and non of my batteries are over 90 Ah.

With the Jag battery being charged on the car even after 3 days voltage 12.8, it never raises over that voltage for more than a few minutes, if the battery voltage drops to 12.7 it moves from 0.1 amp to 0.8 amp until it hits 14.4 volt then drops to 0.1 amp again.

The 12 Ah also switches off at 14.4 volt when on 0.1 amp charge and on again at 12.7 volt.

It is the 35 Ah which has a problem be it on 12 Ah setting or 110 Ah the voltage is held at 13.8 - 14.4 volt for days, this may be no problem, and then again it may be a problem, and this is the question. With a 0.1 amp charge rate is being held at 14.4 volt a problem?