Does your alternator booster overcharge your batteries?

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Oily Rag

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Just a simple fix from a Pragmatic/Practical Bodger & Owner. ( PBO)

Alternator boosters are a bit old fashioned nowadays, but if you have one, this may be of interest.

Mine is tucked out of sight under the quarterberth next to the batteries and in itself, gives no audible alarms. Just lights that flash unseen for most of the time. I recently became aware that it was going into full boost mode of almost two hours at 14.8 volts every time I started the engine. No real problem if I only run for half an hour and then put the sails up. But after the two hour boost finished, I was getting audible alarms from the engine control panel and failrly high voltages on the batteries. (I suspect it had taken the engine battery temporarily to just over the 14.0 volts that the alternator reverts to when not on boost.)

The simple solution of pulling the fuse on the extra field wire that is connected to the alternator was successful in stopping the overcharging, but left me with a continuous audible alarm. I don't know how it manages to activate the engine control panel alarm, but I suspect the installer has put a relay on or in parallel with the extra field wire and connected the alarm buzzer to earth through the relay.

But not wishing to strip the wiring loom and the panel out, I simply put a switch on the power supply to the booster unit to prevent the booster operating at all. Sadly, I got the same result with the alarm buzzer.

Now for the bodge to prevent overcharging. Start the engine with booster switched off and tolerate the noise for a few seconds while the alternator kicks in at about 14 volts. Then switch power to the booster. It starts up, sees plenty of volts and doesn't go into boost mode. Silence!

I still don't know the details of how the booster is wired to the alarm circuit, but until I have time to strip out a snakes wedding of cables, this does the job. It may be useful to others who are still using boosters.
 
Don't know if it applies to yours - but I regard Boosters as when people had twin diode setups and had to boost that 0.7V to overcome the voltage loss across the diodes. So if diodes are removed - the booster should be as well.

Only other I heard of was for getting that last few % of charge into batterys .... but not so common ??
 
It's an X-Alt model by a company called Driftgàte, which ceased trading some years ago. From what I've seen of other manufacturers of boosters, they all seem to operate on a similar principle.

The alternator feeds Driftgate's X-Split, which is, I think based on MOSFET technology. It seems to do the same as diodes, but with a lower voltage drop.

Both a bit out of date, but they work well enough given the above bodge.
 
Oily Rag said:
It's an X-Alt model by a company called Driftgàte, which ceased trading some years ago. From what I've seen of other manufacturers of boosters, they all seem to operate on a similar principle.

The alternator feeds Driftgate's X-Split, which is, I think based on MOSFET technology. It seems to do the same as diodes, but with a lower voltage drop.

Both a bit out of date, but they work well enough given the above bodge.
Click to expand...

The X-Alt has two voltage settings, 14.8v for wet lead acid batteries and 14.4v for AGM/gel batteries. If you're concerned about the charging voltage, why not just select the AGM/gel setting?

The X-Split specification says that the voltage drop is only 15-80mV.
 
If there is any way of selecting lead acid or AGM, then I can't see it on the outside of the booster. I'm reluctant to take it apart to find any DIP switches because it works and I know the power transistors need to be reattached with a conductive paste. The 14.8 volts is, I think, for lead acid so I'll keep that as it is for now.

The batteries are: one sealed SD 6110L on the engine and one of the same on domestics connected in parallel to an unsealed 663. All are lead acid and the same size. 110 Ah. I assume the 663 was originally the engine battery with two 6110Ls for domestics, but I may be wrong. The engine battery is usually about 02 volts above the domestics.

It's possible that for the last five years, the booster has been giving both sets a 14.8 volt hammering for the first two hours if I run that long. (Usually only 45 minutes). At the end of the two hours, the booster stops this and reverts to a normal alternator. At that point, if the engine battery is above the 14 volts that the alternator reverts to, I get an alarm.

All this is because the booster decides at engine start whether it is going to give a boost charge for two hours or not. I mentioned last month that my B+ on the alternator is at 0.35 volts above battery negative even with the engine off. I reckon the booster thinks the batteries are at 0.35 volts lower every time, hence the booster at 14.8 v. Others may have the same problem.

Anyway, I was just pointing out to anyone with similar overcharging problems that if they'd isolate the booster for the first few seconds, then reconnect, it will see alternator voltage and not bother with a boost. I would isolate the booster permanently if I could, but the alarm wouldn't drive me mad!
 
Oily Rag said:
If there is any way of selecting lead acid or AGM, then I can't see it on the outside of the booster. I'm reluctant to take it apart to find any DIP switches because it works and I know the power transistors need to be reattached with a conductive paste. The 14.8 volts is, I think, for lead acid so I'll keep that as it is for now.

The batteries are: one sealed SD 6110L on the engine and one of the same on domestics connected in parallel to an unsealed 663. All are lead acid and the same size. 110 Ah. I assume the 663 was originally the engine battery with two 6110Ls for domestics, but I may be wrong. The engine battery is usually about 02 volts above the domestics.

It's possible that for the last five years, the booster has been giving both sets a 14.8 volt hammering for the first two hours if I run that long. (Usually only 45 minutes). At the end of the two hours, the booster stops this and reverts to a normal alternator. At that point, if the engine battery is above the 14 volts that the alternator reverts to, I get an alarm.

All this is because the booster decides at engine start whether it is going to give a boost charge for two hours or not. I mentioned last month that my B+ on the alternator is at 0.35 volts above battery negative even with the engine off. I reckon the booster thinks the batteries are at 0.35 volts lower every time, hence the booster at 14.8 v. Others may have the same problem.

Anyway, I was just pointing out to anyone with similar overcharging problems that if they'd isolate the booster for the first few seconds, then reconnect, it will see alternator voltage and not bother with a boost. I would isolate the booster permanently if I could, but the alarm wouldn't drive me mad!
Click to expand...

I think you need a marine electrician to sort your system.
 
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