Bad battery or bad charger?

[caleuche]

Hello electron gurus. I'd appreciate your advice on the following.

The starter batteries in my catamaran are charged from the house bank by a mastervolt Magic 12/12-20A DC-to-DC converter. I recently noticed the converter was putting 15.6 volts into the starboard starter battery for several hours, and later on I saw 16v. Since that didn't look like a healthy voltage to me, I disconnected the charger. The battery then showed 12.8 volts but it dropped overnight, with zero load on that battery, to 12.2. Now it looks stable at 12.4v after using it once to start the engine (still haven't reconnected the charger).

The battery is clearly not holding its full charge. It's a 2 year-old AGM Lifeline used exclusively to start the engine. Too young to die.

I can just go ahead and replace that battery, but my question is, should I blame the battery or the charger? Why was the charger putting 16v? (According to the specs, it shouldn't even be capable of 16v for its output). Was it because the battery was dying or was that what could have killed the battery?

Thanks much,

Juan

[john61ct]

Yes constant voltage higher than Float spec will damage a sealed batt.

Getting up way higher than the Absorb spec voltage will do so much faster and more thoroughly.

A constant DCDC converter always on is a very poor design.

A proper device would only activate when a charge source is active.

Best would be a DCDC charger that drops to Float.

But even a small VSR / ACR, or an Echo Charger would be better if you wanted to save money.

[caleuche]

Thanks John. Looks like I have a bad battery and a faulty charger that may have killed the battery then. Two problems for the price of one.

A couple of clarifying points:
- The charge source is always active (it's the house batteries)
- The design is good. The converter acts as a smart charger. Only that the starboard one stopped working as it should, apparently.

[john61ct]

Originally Posted by caleuche

The design is good. The converter acts as a smart charger.

Says who, just the people who installed it?

Usually the term "DC converter" does not include the regulated Absorb vs Float transition functionality essential to modern usage of the term "charger", nor the "only connected while a charge source is active" (as in engine, genset, solar etc) functionality usually required.

If current is indeed **always** flowing from House to Starter, that is not optimal,

certainly inefficient, but also potentially harmful, at least to battery longevity.

[MJH]

Originally Posted by caleuche

Hello electron gurus. I'd appreciate your advice on the following.

The starter batteries in my catamaran are charged from the house bank by a mastervolt Magic 12/12-20A DC-to-DC converter. I recently noticed the converter was putting 15.6 volts into the starboard starter battery for several hours, and later on I saw 16v. Since that didn't look like a healthy voltage to me, I disconnected the charger. The battery then showed 12.8 volts but it dropped overnight, with zero load on that battery, to 12.2. Now it looks stable at 12.4v after using it once to start the engine (still haven't reconnected the charger).

The battery is clearly not holding its full charge. It's a 2 year-old AGM Lifeline used exclusively to start the engine. Too young to die.

I can just go ahead and replace that battery, but my question is, should I blame the battery or the charger? Why was the charger putting 16v? (According to the specs, it shouldn't even be capable of 16v for its output). Was it because the battery was dying or was that what could have killed the battery?

Thanks much,

Juan

I don't like the idea of the starting battery being charged "from the house battery"...the start battery "should always be charged first" or in conjunction with house battery charging (the idea being you will always be able to start your engine). The Blue Sea Systems Automatic Charging Relay does that for my boat.

~ ~ _/) ~ ~ MJH

[Sailmonkey]

Originally Posted by MJH

I don't like the idea of the starting battery being charged "from the house battery"...the start battery "should always be charged first" or in conjunction with house battery charging (the idea being you will always be able to start your engine). The Blue Sea Systems Automatic Charging Relay does that for my boat.

~ ~ _/) ~ ~ MJH

Actually the ACR charges your start battery from the house, and charges it last after the house battery voltage has been brought up to the point that the ACR is activated.

[MJH]

Originally Posted by Sailmonkey

Actually the ACR charges your start battery from the house, and charges it last after the house battery voltage has been brought up to the point that the ACR is activated.

Blue Sea Systems words:
Manual Battery Management and ACR SubSystem

SI-ACR automatically combines batteries during charging, isolates batteries when discharging and when starting engines. The Dual Circuit Plus™ Battery Switch simplifies the switching of two batteries to OFF or ON while still providing battery isolation between the engine and auxiliary battery. The Combine function can be used to parallel the batteries in the event of a low engine battery.



~ ~ _/) ~ ~ MJH

[Zil]

Was the high voltage only on one starter battery? What was the other start battery doing? Are both charged from the DC/DC ?

[Sailmonkey]

Originally Posted by MJH

Blue Sea Systems words:
Manual Battery Management and ACR SubSystem

SI-ACR automatically combines batteries during charging, isolates batteries when discharging and when starting engines. The Dual Circuit Plus[emoji769] Battery Switch simplifies the switching of two batteries to OFF or ON while still providing battery isolation between the engine and auxiliary battery. The Combine function can be used to parallel the batteries in the event of a low engine battery.



~ ~ _/) ~ ~ MJH

If that’s how your ACR is wired, great. Ours is plumbed to charge the house first and then the start. More charging is required to the house batteries than the start could ever need.

[BruceS]

AGM for engine starting? Not sure if that's the best way to go?

[Marqus]

This Mastervolt Magic seems a versatile bit of kit. Can do a few things, one of which is a 3-stage DC-to-DC charger (see diagram below).

Seems like it should ABSORB around 14.2v and FLOAT around 13.2v.

To check whether the charger is at fault, I would try it on another known good, fully charged battery (any size) and check the Float voltage applied to said battery - if 13.2v that would be a good sign for the Mastervolt.
After that, I would explore the Absorption and Bulk charge behaviour as well. If the Mastervolt behaves, then maybe the AGM batt failed prematurely (not unheard of) though Lifeline's are good stuff so maybe it can be reconditioned via a few cycles.

[john61ct]

Originally Posted by MJH

I don't like the idea of the starting battery being charged "from the house battery"...the start battery "should always be charged first" or in conjunction with house battery charging (the idea being you will always be able to start your engine).

absolutely not

Starters don't get discharged more than .003%

Best practice is all significant sources go direct to House that needs it, then only a little ampacity cheaper device is needed to keep it topped up.

[john61ct]

Originally Posted by BruceS

AGM for engine starting? Not sure if that's the best way to go?

fine, can in fact be better, but NP either way

[john61ct]

so yes, that line is a DC-DC 3-stage battery charger, happens to be settable to dumb down as a DC-DC converter as well.

triple the price needed, you'd think they'd last

[Marqus]

Given the OP's info so far, to me it's a bit early to condemn the Mastervolt charger.

If the battery was in some way deficient, could the charger not have (by design) pushed up to 16v in BULK mode?