Alternator charging query

Cardo

Cardo

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Morning, all

As we're currently motoring down the French canals, we're obviously running the engine for a large part of the day and we don't often have shore power in the evenings. Also, the weather is currently total crud so solar panels are making no difference. It's all on the engine alternator at the moment.

However, I've noticed something interesting. When we first start the engine, the (domestic bank) ammeter shows plenty of amps coming in, however this only lasts a few minutes before it drops to around 5a or so for the rest of the trip.

The alternator is connected to the engine battery and domestic bank via what appears to be a basic splitter.

My hypothesis is that the engine battery is being topped up, then once the high voltage on the engine battery is detected, the alternator is slowing charge.

Yes, I appreciate I really should just get a proper alternator regulator thingy, but it'll have to wait.

I do have the ability to parallel the engine and domestic batteries via a separate switch. Would it be a good idea to do this once the engine is running so hopefully the alternator continues to charge everything? Or do I risk flattening the engine battery and ending up with no workie engine??
 
When you start the engine there is a surge from the batteries, and this causes the terminal voltage to drop, so this is why your output is high, however the battery will soon equalise and re-gain the initial charge, this leads to the terminal voltage rising quite quickly, and the alternator output falls off too soon for effective deep charging.

If you are relying on alternator charge to top the battery up then you need an alternator controller such as a sterling unit. This causes the alternator output to rise and makes the alternator act like a multi stage battery charger.

Good piece of kit an fairly easy to install, with more detailed explanation of why its needed. I have the Sterling details available as PDF - PM me and I can send it to you.
 
Cardo said:
My hypothesis is that the engine battery is being topped up, then once the high voltage on the engine battery is detected, the alternator is slowing charge.
Click to expand...

The alternator does not control amps, it only limits volts, the amps you see is limited by what the battery can except at your running voltage.

You need to check charging voltages at alternator, plus at engine and service battery terminals, then come back.

It could be alternator regulator voltage, could be the split charge system, could be a bad connection.


Brian
 
OP obviously has plenty of engine run time so one would not imagine a stepped alternator charge controller should be needed. What is happening is quite normal however if the domestic batteries were well discharged one would expect the charge rate to be higher for longer.
What OP should do is check the voltage on the battery terminals when the engine is charging. If this is around the 13.75 to 14v then you are doing as well as can be expected. However if there is a pair of diodes as splitters then these may well be reducing the voltage to the batteries so charge current is reducing quicker. If you measure more like 13v on charge then that would be the problem. Alternator is sensing voltage at its output not at the battery +ve. The batteries will never be really charged.
If this is the case and you can find the splitter it will have a heat sink and 3 studs on it. Just short all 3 studs together. This will leave alternator output connected directly to the engine and domestic battery positive. Note that the short across the studs should be removed when you stop the engine to maintain isolation of the engine and domestic battery.
If this seems to improve charge and brings battery voltage up to normal then perhaps the cheapest easiest option is to fit a Voltage sensing relay. Thus the engine battery will always be connected to the alternator output and the domestic battery will be charged once engine battery reaches about 13v when the relay will operate and connect the domestic battery to the alternator output.
If on the other hand the battery charge voltage looks good then either the domestic batteries were not as discharged as you thought or have lost capacity. In which case you will notice they don't provide power for as long as you would expect.
good luck olewill
 
The original alternator on my boat was battery sensed: that is, it set its output voltage by measuring the voltage in the domestic batteries. It had a diode splitter and the system worked fine. Then it died. The replacement alternator is not battery sensed and I found the same problem as the op. I removed the diode splitter and fitted an isolation switch on the output to the engine battery. This gives me better charging rates on the domestic bank. This is because the engine battery is rapidly topped up, the alternator sense this and cuts back the voltage. The only downside is remembering to switch the engine battery into the circuit for the first 20 minutes or so and the switching it of again to isolate it and protect the charge level for engine starting.
 
Without a smart controller the best level of charge one can expect in your batteries is about 80%. As already suggested you would do well to fit an Adverc or Sterling external controller.
As overcharging is the worst fate that can befall your batteries, the slight undercharge you're experiencing is erring on the side of caution.
 
Agreed. My concern when we went through the French canals was overcharging, but when we finally reached the Med the batteries didn't even need topping up!
 
If after the checks WilliamH suggests, the splitter is the culprit, I'd replace it with a relay. The experts will say it ought to be a voltage sensitive one, but I've been using the relay from my old dynastart fir 8 or 9 years without any problems. An ordinary car relay would be fine; I'd look for one that can switch the full output of your alternator, but it'll never have to do that, so I wouldn't lose any sleep over using a 55 amp one (a common size) with a 70 amp alternator. Here's an example, though 30 amps is a bit on the light side:

080_081.jpg


Connect contact 30 to the output from the alternator, which also needs to go to the starting battery, 85 to the wire that runs from the charge warning light and the field connection on the alternator, 86 to earth and 87 to the domestic battery.
 
Thanks for the replies, all.

I think it may simply be a case of the alternator turning down at around 80% so if the batteries are pretty low it will take an age to get past that on a trickle charge. I'll leave it as is and get a proper regulator once the budget is authorised!
 
Cardo said:
Thanks for the replies, all.

I think it may simply be a case of the alternator turning down at around 80% so if the batteries are pretty low it will take an age to get past that on a trickle charge. I'll leave it as is and get a proper regulator once the budget is authorised!
Click to expand...

May I suggest again you check the three voltage levels, could save your bank balance some money.

Brian
 
Stemar said:
If after the checks WilliamH suggests, the splitter is the culprit, I'd replace it with a relay. The experts will say it ought to be a voltage sensitive one, but I've been using the relay from my old dynastart fir 8 or 9 years without any problems. An ordinary car relay would be fine; I'd look for one that can switch the full output of your alternator, but it'll never have to do that, so I wouldn't lose any sleep over using a 55 amp one (a common size) with a 70 amp alternator. Here's an example, though 30 amps is a bit on the light side:

080_081.jpg


Connect contact 30 to the output from the alternator, which also needs to go to the starting battery, 85 to the wire that runs from the charge warning light and the field connection on the alternator, 86 to earth and 87 to the domestic battery.
Click to expand...

You the man ;-) I'll keep that info

cheers
mick
 
mtb said:
You the man ;-) I'll keep that info

cheers
mick
Click to expand...

This concept of connecting the relay to the warning light might work Ok but is predicated on using a low current high resistance relay coil. If you use a big relay with higher current coil then the coil will in effect short out the regulator and field coils so stopping the alternator from initially charging. to explain.
So you get 12v from the ignition switch this goes through the alternator light to the regulator then field coils. (or the other way round coils then regulator) Hence when you turn on ignition before starting you have a voltage at the lamp 12v on one side and quite low on the other due to current through regulator and field coils. So with the relay connected from cold or regulator side of lamp to ground there is not much voltage so relay does not operate. (so no connection start to domestic battery).
When the alternator starts to charge. The regulator and filed coils stop taking current via the warning lamp. They have their own supply. Hence relay operates with positive 12v via the lamp to the relay. The lamp may glow a little with relay current.
With a larger current relay lamp will glow a lot at all times and relay will suck current from the regulator/ field coils. The symptoms would be slow to start charging.
It is all about actual resistance of the relay coil and the current needed by the alternator for initial excitation. I can't help thinking Stemar was lucky to get that relay drive to work.
I would rather go to a voltage sensing relay or a relay operated by oil pressure light switch. Or if you want to use thee ignition light circuit use a tiny high resistance coil relay to operate a bigger relay. (one that could also provide jump start capability)
Of course I may be wrong olewill.
The term ignition and ignition light come from their use in petrol engines where it also operates the ignition. In diesels of course it is just for alternator excitation and guages but still seems to be called "ignition".
 
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