Batteries Only Accepting Low Amperage

[nhschneider]

We have eight 6-volt AGM golf cart batteries, about 220 amps each, wired to make 12 volts. Our ‘genset’ is a single-cylinder diesel with a 150-amp alternator and an external Balmar regulator. Everything has been working fine for many years and many thousands of miles.

However, now when the batteries are low (down to about 12.2 volts) and I start the genset, I can see the amps start off at about 100 amps per hour but then quickly drop down to about 50 amps. Putting 200 amps back into the batteries at 50 amps per hour is a lot of genset run time. The batteries test fine when I conduct a load test. Any ideas?

[Knot Safety]

I’m guessing an alternator issue.

[Stu Jackson]

What makes you think it was any different before? Had you recorded those values, too?


All charging sources will ramp down like that.


You also might want to brush up on your use of amps and amp hours, before The Other Stu (i.e., StuM) shows up.

[S/V Illusion]

Understand a fundamental principle - batteries can accept only a fraction of their capacity per hour. Generally around 20%. And as they rise in voltage, the charge rate is supposed to decrease if your charging system is operating properly.

Regardless of how hard you try, you can’t change their acceptance rate without cooking them at any SOC condition.

No charger regardless of rating can pump more into them than they are capable of accepting safely.

[Dsanduril]

Setting aside failing batteries (could be that, but not enough information), if this is newish behavior and/or a slow degradation - they used to charge faster but now do not - it could be a corroded connection anywhere in the circuit between the alternator and the batteries.

Assuming you are using voltage regulation measured at the alternator, any dirty connection between the batteries and the alternator can cause a voltage drop that makes the alternator think the batteries are fuller than is actually the case. If your alternator uses remote sensing (a separate voltage sensing wire) this could still be the problem, although in that case the bad connection could be in the sensing wire (not the highest likelihood) or in the main negative cables (more likely - since most remote sensing is measured between a single remote positive wire and the case ground on the alternator). If you have a two-wire remote sensing then I'm probably completely off base.

You can check this with a volt meter (possibly with very long leads). While generating (be very careful) measure the voltage between the battery + terminal and the alternator + terminal. In an ideal world this would be zero, but it will be some non-zero number. The higher the number the greater the losses in your cabling. If it is getting into the tenths of Volts then something to look at. Repeat with the measurements from negative battery terminal to alternator case (or alternator ground connection if it has a separate ground connection). If at all possible, when taking these measurements you want to connect your voltmeter in a way that will account for the connections at the battery and at the alternator - otherwise these could be the problem and you might miss a bad connection, but this can be difficult to achieve.

If you do find a voltage drop in either cable then start looking at voltage drop at other connection points, i.e. battery + to battery switch, battery switch to alternator, bus bar or fuse or shunt to ..... You're basically looking to see if you can identify a single connection with a significant voltage drop. Best time to do this measuring would be once the charge current drops to 50A or whatever the value is that you see as a problem, that will have you testing at the conditions that represent a problem, will give a bad connection time to heat up and generate resistance, etc.

Another tool for this, non-contact, is an IR thermometer. Check every single connection without charging, then check again once you've been charging for a while, if you find connection(s) that significantly increase in temperature that's your culprit. Don't forget the negative side, it often gets overlooked but it is half the circuit.

[nhschneider]

Okay, I realize I should have explained myself better.

Previous to now, with the exact same components, when the batteries were about to the same level of discharge, when I would start the genset, I would see about 120 amps of charge that would slowly drop to about 50 amps over the course of about an hour and ten minutes. Now I see initially about 100 amps, tapering off to about 50 amps in about five minutes. Clearly, the alternator has the ability to put out the amps but, for some reason, is reducing the amps far too quickly.

I like to think that I have an adequate knowledge of marine electrical systems and I certainly can read and understand acceptance rate graphs but this problem has me stumped.

Thank you for your replies.

[Sailmonkey]

How old are your batteries?

How hot is the alternator when this happens?

What is the battery voltage when this happens?

Measure the voltage @ the batteries with a multimeter while charging.

[CarlF]

It sounds like you may be yet another victim of AGM PSOC ("Partial State of Charge") failure.

AGM's have to be brought up to 100% charge frequently. If not they can fail in just one year. To get to 100% you either need to plug in to a marina or have a huge solar array. The slow acceptance from 95% to 100% takes many hours of genset running which no one wants to listen to (nor is it good for the genset to run so lightly loaded).


I would try charging each pair separately and see which take a reasonable charge. It's possible that only some pairs have been badly damaged. This would also uncover other possibilities such as bad cables or corrosion. Be sure to measure each pair's voltage. If they are more than .2V different, that is a bad sign.

You can search AGM PSOC to find many who have been in the same boat - including me. I switched to Firefly batteries after my AGM's failed just short of 3 years despite lots of care. Firefly's are not subject to PSOC. Another option is to go to traditional flooded, gel, or lithium.

[nhschneider]

CarlF-
Thank you for your reply. That explains perfectly what might well be the problem. After we installed the new AGM batteries, we sailed from Norfolk, Virginia to San Diego, California, and rarely spent any time in a marina. Nor is our solar array large. The batteries were virtually never at 100% charge level.

Not that I’m pleased to have to change out the batteries but I am relieved to have a logical explanation. Thank you again.

Fair winds and calm seas.

[sailorboy1]

Originally Posted by nhschneider

Okay, I realize I should have explained myself better.

Previous to now, with the exact same components, when the batteries were about to the same level of discharge, when I would start the genset, I would see about 120 amps of charge that would slowly drop to about 50 amps over the course of about an hour and ten minutes. Now I see initially about 100 amps, tapering off to about 50 amps in about five minutes. Clearly, the alternator has the ability to put out the amps but, for some reason, is reducing the amps far too quickly.

I like to think that I have an adequate knowledge of marine electrical systems and I certainly can read and understand acceptance rate graphs but this problem has me stumped.

Thank you for your replies.

Its the batteries. Time to replace. Thats the way way it is.

[smac999]

what is the battery voltage when it ramps down? (measured at battery) and also what is the BV and CV reading on the balmar display.

if the BV and CV is say is at 14.4v and so is the battery. and it's droping amps. the batteries likly are going bad. (assuming they used to take more but a 900ah agm bank should probably not be droping after an hour of charging starting from 60%)

if the BV and CV is 14.4v but the batteries are 13.8 or something at the battery then your regulator wiring is bad and you're probably reading off the back of the alt and not the battery. this will cause it to ramp down to early.

if it's bv is 13.5v and cv is 14.4 and the current ramping down. it's an alternator issue or heat issue.

[Kd9truck]

Great thread!! I just did a read through and learned some “things” and affirmed theoretical knowledge. I bought batteries last year (12v AGMs), they replaced batteries with similar readings as yours....it was time for new batteries for me. This post affirms my conclusions, I changed out my alternator first. Now my batteries get toped off every-time I come off mooring, and the diesel likes the regularity.

[Gary Matthesen]

Originally Posted by Dsanduril

Setting aside failing batteries (could be that, but not enough information), if this is newish behavior and/or a slow degradation - they used to charge faster but now do not - it could be a corroded connection anywhere in the circuit between the alternator and the batteries.

Assuming you are using voltage regulation measured at the alternator, any dirty connection between the batteries and the alternator can cause a voltage drop that makes the alternator think the batteries are fuller than is actually the case. If your alternator uses remote sensing (a separate voltage sensing wire) this could still be the problem, although in that case the bad connection could be in the sensing wire (not the highest likelihood) or in the main negative cables (more likely - since most remote sensing is measured between a single remote positive wire and the case ground on the alternator). If you have a two-wire remote sensing then I'm probably completely off base.

You can check this with a volt meter (possibly with very long leads). While generating (be very careful) measure the voltage between the battery + terminal and the alternator + terminal. In an ideal world this would be zero, but it will be some non-zero number. The higher the number the greater the losses in your cabling. If it is getting into the tenths of Volts then something to look at. Repeat with the measurements from negative battery terminal to alternator case (or alternator ground connection if it has a separate ground connection). If at all possible, when taking these measurements you want to connect your voltmeter in a way that will account for the connections at the battery and at the alternator - otherwise these could be the problem and you might miss a bad connection, but this can be difficult to achieve.

If you do find a voltage drop in either cable then start looking at voltage drop at other connection points, i.e. battery + to battery switch, battery switch to alternator, bus bar or fuse or shunt to ..... You're basically looking to see if you can identify a single connection with a significant voltage drop. Best time to do this measuring would be once the charge current drops to 50A or whatever the value is that you see as a problem, that will have you testing at the conditions that represent a problem, will give a bad connection time to heat up and generate resistance, etc.

Another tool for this, non-contact, is an IR thermometer. Check every single connection without charging, then check again once you've been charging for a while, if you find connection(s) that significantly increase in temperature that's your culprit. Don't forget the negative side, it often gets overlooked but it is half the circuit.

Thankyou Dsanduril, I too enjoyed reading through your thread, some great "diagnostic" ideas I will share with friends.

[jamhass]

Neil:

Check out MaineSail's site on programming the Balmar. May be that it is "throttling" down too quickly.

https://marinehowto.com/programming-...age-regulator/

[David B]

Originally Posted by nhschneider

CarlF-
Thank you for your reply. That explains perfectly what might well be the problem. After we installed the new AGM batteries, we sailed from Norfolk, Virginia to San Diego, California, and rarely spent any time in a marina. Nor is our solar array large. The batteries were virtually never at 100% charge level.

Not that I’m pleased to have to change out the batteries but I am relieved to have a logical explanation. Thank you again.

Fair winds and calm seas.

Consider decent FLA's. Half the price, half as fussy, and last nearly as long (8 years or so). I keep doing the maths, and unless you have a special need, FLA's keep winning the $'s vs Ah's vs yrs race.