Bulk vs Absorb Voltage

[Trevor NZ]

Hi all

I have a Mastervolt Chargemaster 75-3, i.e 75A. House Bank is 3x Remco 214ah AGMs"12-200 DC" in parallel at 12v. With some exceptions the boat is (unfortunately!) mainly used for a few nights then back on shorepower at the dock. It has a tiny draw at the dock - one small 12v fridge. Perhaps 2A/hour max.

Mastervolt charger defaults to 14.4v bulk, 14.25v absorb and 13.8v float for its AGM charging profile. Charger has temp probe which we've connected.

I've now got a masterbus unit so can fine tune parameters on laptop.

Batteries are stamped with a label stating 13.5-13.8v for "standby use" and 14.4-15v for "cyclic" use.

My question is what voltage to use for bulk, absorb and float?

Currently I have them at:
14.7v for bulk ("start time" at 13.25v),
14.3v for absorb - this is the one I'm a bit confused on. Most places seem to suggest same voltage for bulk and absorb but Mastervolt charger default has it a bit lower than bulk. Return amps is actually at Mastervolt default of 6% which is 4.5amps (6% of 75A) equating to 0.7% of bank capacity which may be a bit aggressive but they are newish batteries showing 100% SOH on the Balmar SG200.

Should I lift absorb to 14.7v as well?

Float: 13.65v being midpoint of the battery's "standby use". Return to bulk is at 12.8v.

Grateful for your thoughts.

Thanks

[PaulCrawhorn]

Standby means not cycled, like a UPS.

With the use you describe lower voltage is better than high, and Bulk can be equal to Absorption, my vote sitting on shore is for 14.2 and 13.5 for Float.

When you go out if you think of it, try to give them a good draw down, but not below 50%, and then bump any charge source up to 14.8V for recharging at least 8 hours.

It won't hurt them if you leave it like that since they just Float on mains anyway, but best to turn it down if you can remember.

Return amps could be at half a percent of AH capacity when finishing Absorption, no harm not "aggressive" good for them.

Return to Bulk is fine, sounds like it never happens anyway.

[StuM]

Originally Posted by Trevor NZ

It has a tiny draw at the dock - one small 12v fridge. Perhaps 2A/hour max.

Again! 2A/hour is essentially meaningless. Please learn the correct untis to use when discussing electricity.



Do you mean it draws 2A when runnning or do you mean it generally uses less than 2AH per hour?

[StuM]

Originally Posted by Trevor NZ

My question is what voltage to use for bulk, absorb and float?

Currently I have them at:
14.7v for bulk ("start time" at 13.25v),
14.3v for absorb - this is the one I'm a bit confused on. Most places seem to suggest same voltage for bulk and absorb but Mastervolt charger default has it a bit lower than bulk. Return amps is actually at Mastervolt default of 6% which is 4.5amps (6% of 75A) equating to 0.7% of bank capacity which may be a bit aggressive but they are newish batteries showing 100% SOH on the Balmar SG200.

Should I lift absorb to 14.7v as well?

Float: 13.65v being midpoint of the battery's "standby use". Return to bulk is at 12.8v.

Grateful for your thoughts.

Thanks

14.7 for bulk means that the charge will throw everything it can at the batteries until the reach 14.7V.

Once they reach that, the charger will throttle back so that the battery voltage drops back to 14.3V and holds at that until the charge rate falls bel0w the cut off. That can make sense with FLA since the lower voltage for longer time should result in less off-gasing so less frequent water top-ups.
I'm not sure that there is any advantage in lower absorption voltage with AGM, especially if you have a temeperature monitor since the only other poential problem is overheating.

[smac999]

There is no such thing as a bulk voltage.

Bulk stage is current based untill the absorbsion voltage is reached.

Balmar has made their own bulk definition and it seems mastervolt has now too.

If these are sealed batteries I would set bulk and absorb to 14.4 and float to 13.5. If they are refillable flooded I would set the bulk and absorb to 14.7

The charger amps are meaningless. Turn on lights and fridge abd the charger is putting out 10a even if the batteries are full. And the charger will never go lower untill loads are turned off. Absorb charging is always timer based because of this.

[PaulCrawhorn]

> There is no such thing as a bulk voltage. Bulk stage is current based untill the absorbsion voltage is reached. Balmar has made their own bulk definition and it seems mastervolt has now too

Dozens of engineers at very well respected vendors use the terminology "wrong", add Trojan to your list.

Sometimes even reversing the order, Bulk used for CV stage Absorption for the prior CC.

Many say that GEL batteries actually benefits from a lower voltage in the last few hours finish stage, and OK to speed the cycle up a bit with a higher voltage at lower SOC.

So yes in principle you are absolutely correct, Bulk "should" mean CC stage no fixed voltage, Absorb is after the CV setpoint is reached.

Myself I agree with you, just one charging voltage is enough, and then Float, only after 100% SOC has been reached.

In reality, normal usage, I think it is best to simply state, "Set the Bulk voltage to the same as Absorb, or as close as possible".

[noelex 77]

Early charge algorithms incorporated a bulk voltage that was slightly higher than the absorption voltage. Hence on the better battery chargers and alternator and solar regulators it was possible to set different bulk and absorption voltages.

Many modern charge devices have simplified this and it is not possible to set different bulk and absorption voltages.

Unfortunately, there is very little information about the benefits and drawbacks of a higher bulk voltage.

Personally for most lead acid battery chemistries I take advantage this ability to set a higher bulk voltage when it is available, but I can only justify this with a gut feeling rather than hard science.

A slightly higher bulk voltage can also be used (albeit rather crudely) to help mitigate the effects of voltage drop in the wiring.

The best starting point is to use the recommendations from the manufacturer for cyclic use. If they give a range then err on the higher side.

This voltage should be corrected for temperature and also confirm that this voltage is actually reached at the battery. Voltage drop can have a significant effect.

If the manufacturer lists a single bulk/absorption voltage (as most do), either use this as your absorption voltage and set the bulk voltage to same number, or use this for the absorption voltage and set the bulk voltage slightly higher (typically around 0.1v ).

[noelex 77]

Originally Posted by PaulCrawhorn

> There is no such thing as a bulk voltage. Bulk stage is current based untill the absorbsion voltage is reached.

Bulk voltage is real.

The above should read:
The bulk stage is current based untill the bulk voltage is reached

You can make the bulk and absorption voltages identical or different.

If the voltages are identical (which is the only choice on some regulators) you can call this set point the bulk voltage, the absorption voltage or the bulk/absorption voltage.

[MJH]

Originally Posted by Trevor NZ

Hi all

I have a Mastervolt Chargemaster 75-3, i.e 75A. House Bank is 3x Remco 214ah AGMs"12-200 DC" in parallel at 12v. With some exceptions the boat is (unfortunately!) mainly used for a few nights then back on shorepower at the dock. It has a tiny draw at the dock - one small 12v fridge. Perhaps 2A/hour max.

Mastervolt charger defaults to 14.4v bulk, 14.25v absorb and 13.8v float for its AGM charging profile. Charger has temp probe which we've connected.

I've now got a masterbus unit so can fine tune parameters on laptop.

Batteries are stamped with a label stating 13.5-13.8v for "standby use" and 14.4-15v for "cyclic" use.

My question is what voltage to use for bulk, absorb and float?

Currently I have them at:
14.7v for bulk ("start time" at 13.25v),
14.3v for absorb - this is the one I'm a bit confused on. Most places seem to suggest same voltage for bulk and absorb but Mastervolt charger default has it a bit lower than bulk. Return amps is actually at Mastervolt default of 6% which is 4.5amps (6% of 75A) equating to 0.7% of bank capacity which may be a bit aggressive but they are newish batteries showing 100% SOH on the Balmar SG200.

Should I lift absorb to 14.7v as well?

Float: 13.65v being midpoint of the battery's "standby use". Return to bulk is at 12.8v.

Grateful for your thoughts.

Thanks

I would contact the battery manufacturer and ask for the specific recommended settings for Bulk, Absorb, and Float and set my battery monitor & regulator to those settings.

~ ~ _/) ~ ~ MJH

[sailcrazy]

Following.

[Flatswing]

Originally Posted by MJH

I would contact the battery manufacturer and ask for the specific recommended settings for Bulk, Absorb, and Float and set my battery monitor & regulator to those settings.

~ ~ _/) ~ ~ MJH

2nd the above: (Marinehowto.com) suggests the same. Many battery manufacturers give a range, rather than an isolated number. If so, since you are plugged in mostly, use the low end of the range for float.

[Pelican_38]

Originally Posted by MJH

I would contact the battery manufacturer and ask for the specific recommended settings for Bulk, Absorb, and Float and set my battery monitor & regulator to those settings.

~ ~ _/) ~ ~ MJH

I see the wisdom in this comment - but sometimes the experts leave me scratching my head.

I did just as you recommended a couple weeks ago and contacted Interstate Batteries. One of their engineers sent me a data sheet on lead acid, group 31, deep cycle battery and it stated:
Bulk - 14.4v
Absorb - 15.8 - 16.0v
Float - 12.9 - 13.2v
Equalize 16.0 - 16.3v

I questioned him about the absorption and mentioned it was much higher than I expected and wondered if that was a typo and was actually the equalization voltage. He said "nope, you are OK to set your charger to these parameters".

I'm not sure I'm going to set my absorption voltage to be higher than bulk, that just doesn't pass the makes-sense test.