Battery Charging, Alternator vs Charger

[TugboatEng]

I maintain a fleet of boats and have noticed that the boats with alternators on the engines do not get the same life out of batteries as the ones without alternators. All batteries have wall mount chargers. The wall mount chargers have bulk and float charging modes, some had absorption cycles too but we don't have any more of those chargers. Do alternators run in as only bulk chargers all of the time and does that shorten batter life on engines that rack up a lot of hours (3000-4000 hours a year)?

 

[itsmoked]

Definitely. Alternators are tilted towards getting the starting hit recovered to the battery ASAP and most modern vehicles have lots of loads, so again, the alternators are setup to be aggressive. You have a boat that might not have a lot of electrical load (could.. but could not) also it may be started and run at a high speed for most the day. All the alternators I've ever seen always put out a 'cyclical charge voltage'. I never see a "float" voltage out of alternators. Constant cyclical results in over charging. On the upside that helps remove sulfating so the batteries can keep their capacity, but long term they're frequently overcharged.

Another even more impactfull issue is the temperature/voltage curve that dictates you charge to a lower and lower peak voltage as the battery temp goes up. Theoretically that compensation is either in the alternator regulator or inherent in the normal reduced output with temperature increase of all alternators. If your boat batteries are removed from the alternator's ambient that compensation could be worse than usual.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TugboatEng]

Our DC electrical loads are minimal. A 2500 hp electronic controlled diesel engine only draws about 10 amps and 24 volts at full load, total load is about 20 amps peak with the Z-drive load included. Power is backed up by two 8D batteries in series. The rest of our alternator engines are 150-175kw gensets so they run at 1800 RPM all day and have a single single 8D battery for starting. These are the worst in our fleet for short battery life. They are common rail diesel engines and probably have a running load of around 5 amps at 12v.

I don't believe we have temperature compensation for our chargers. They are generally located in the same space as the batteries. Most of our fleet runs magnetic (transformer based chargers) which I believe are the best money can buy though they don't have many "features".

 

[itsmoked]

2500hp! Wow. That's a thrust maker. The last locomotive I worked on was only 1500hp.

We can help figure out an improvement but there are several things that can all lead to early battery death and one needs to know which one or sometimes 'ones' are the culprits.

I just bought a new Digital MultiMeter called an iDVM510 by RedFish Instruments that does everything any other DMM does except it also does data logging - all for $149. You could slap one of those on your battery bank and let it log for 24 hours then dump it to us and we'd likely see EXACTLY what's going on. (Oddly I got it at Home Despot - though the meter is produced in my town.)

Alternatively you can proactively switch to any one of several good 3 stage mains powered chargers and ditch the alternators and their fairly crummy battery care. However a very common problem is deep cycling starting batteries which can occasionally be happening without your knowledge. Every night with the engines not running some parasitic load (i.e. bilge pump) could be taking the top off the battery charge so the next day a full-on recharge has to take place. Healthy battery life is totally cycle based and depth-of-cycle related. A battery that is used to start an engine only and promptly recharged will last years longer than one that is discharged every night 10 or 20% and recharged the next day. Putting a data logger on the bank would expose that issue quickly. The entire charging aspect could be just fine it's just the discharges wearing them out.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Tug; One other good possibility is purely the ambient temperature.

All Lead-Acid batteries are spec'd at 25C° or 77F°. Most are rated at about 7 years lifetime based on their sulphating and/or corrosion lifetimes.

For every 15F° over that 77F° the average battery life is reduced by half. That means a 7 year battery will be expected to fail in 3.5 years in an average ambient of 92F°. I've been in a lot of boat engine rooms that easily averaged that or higher. Could it be your two generator batteries are in much warmer spaces than the prime mover batteries?

Keith Cress
kcress -

http://www.flaminsystems.com