AGM battery - 18 volts!!! Eeek.............

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Ronald_M

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.............Perhaps I might explain....?

I've just treated my little trailer-sailer to a swanky new electric-start outboard complete with a DC charging output rated at 70w/12v. This voltage will, of course, vary with engine speed and also the internal resistance of the battery the engine is connected to.

The manufacturer recommends that the engine should be connected to a "standard" 40aH automobile battery. I am assuming that this arrangement would be pretty much self regulating - as the battery charges up, its resistance increases and the charge voltage/amperage decreases?

Being a fan of AGM batteries, however, I have attempted to link the engine up batteries of that variety. I've tried two different sizes - a 100aH and a 5aH (a small motorbike battery)

Connecting a handheld voltmeter across the battery terminals shows charging voltages of:

100aH battery - 16.5 volts max
5aH battery - 18.2 volts max

Probably result - fried batteries.

I have been informed that no suitable " intelligent" charge regulator able to intermediate between my engine and a battery of the AGM type exists.

Or does it? Anybody know?

TIA

Ron.
 
as a rough and ready sort of fix, about 20 metres of cable between the outboard and the battery should produce about a 2 volt drop which should keep the battery charging ok
 
TopDonkey said:
Actually, even better is to use a solar panel charging regulator, it would be perfect for the job !!

http://cgi.ebay.co.uk/180-WATT-16-AMP-SOLAR-PANEL-DUAL-CHARGING-REGULATOR-/110697088382?pt=UK_Gadgets&hash=item19c60f857e#ht_2421wt_2475
Click to expand...

No I think you wll find the Brocott regulator may just switch the charging off at 14.4 volts. No problem with a solar panel but with inductive devices like the outboard charging coil a big surge in volts can result and damage the outboard's rectifier.

outboard regulators are shunt type regulators some even water cooled but they are usually only fitted to larger outboards with higher output charging coils

No regulator is necessary. You probably wont run the engine for long enough or fast enough to overcharge the battery.

My outboard is connected directly to the battery. At one time a car starter battery , then a 45Ah leisure battery, now a 60Ah leisure battery.
Far from overcharging, until I bought a solar panel I still had to take the battery home for charging now and then.

Search for a regulator for a motor cycle if you really must fit one !

Do what the outboard manufacturer recommends. Use a bog standard small automotive battery. Dont waste money on anything else.
 
i would have thought the rectifier would have been robust enough to withstand disconnection whilst running ?, its only a 70 watt output after all, but if it isnt, you can just put a high power resistor across the input to dampen the back emf surge as the coil is disconnected
 
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TopDonkey said:
i would have thought the rectifier would have been robust enough to withstand disconnection whilst running ?, its only a 70 watt output after all, but if it isnt, you can just put a high power resistor across the input to dampen the back emf surge as the coil is disconnected
Click to expand...

Those who know better than I caution against it for the reasons given.
They know it can blow the rectifier .. you only think it will be alright.

No sense in the OP risking the damage to his new engine (and outboard rectifiers dont come cheap) when there is no need to!
 
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Charge voltage

I find it surprising that you get such a high charge voltage.
Is your volt meter correct? Digital VM tend to read high when the internal battery is low.

I would have thought the battery would soak up the charge so keeping the voltage down until it is overcharged or boiled dry. You could try the meter on amps scale and measure current going in to the battery. (buy a new 9v battery for the meter first.)
If it really is overcharging then you could try a simple regulator in the form of a Zener diode rated at 14 volts and as many watts as you can find. Fit a series resistor of about 20 ohms for a 1 watt zener.
This device then will dissipate 70 milliamps when the voltage rises above 14v. You could fit more zener/resistors in parallel to dissipate more current. In dissipating the current you will reduce the charge voltage a bit. Better is a zener as a reference allowing a power transistor to conduct wasting the current. In other words a shunt regulator just like a solar regulator.
Or just run some lights if you are running the engine for a long period. good luck olewill
 
I agree with William H -
Change your multimeter battery , even before the low battery icon appears a meter can over read by several volts.

I have gone back to using an old AVO analogue meter .
 
Thanks for all your thoughts, chaps.

It hadn't occurred to me that a flat multimeter battery might give erroneous readings; I assume it's probably OK as the meter shows voltages of about 12.5 - 13.5 when the batteries are "resting". However, I'll shove in a brand new PP9 asap anyway!

My current thinking (excuse the pun) is to go with the manufacturer's recommendation and fit an ordinary wet-cell 40aH car battery. My understanding is that AGM batteries have a lower internal resistance - perhaps explaining the high charge voltages - whereas conventional flooded-cell ones would offer a bigger loading to the charge coil. This might be all that is needed to regulate things, but in any event, conventional batteries are supposed to be more tolerant of overcharging. Providing I remember to keep an eye on the electrolyte level, everything - hopefully - will be OK. Er, hopefully!

Thanks again,

Ron.
 
Ronald_M said:
My understanding is that AGM batteries have a lower internal resistance - perhaps explaining the high charge voltages - whereas conventional flooded-cell ones would offer a bigger loading to the charge coil. This might be all that is needed to regulate things, but in any event, conventional batteries are supposed to be more tolerant of overcharging. Providing I remember to keep an eye on the electrolyte level, everything - hopefully - will be OK. Er, hopefully!

Thanks again,

Ron.
Click to expand...

AGM batteries have lower internal resistance and will take higher charge currents initially.
However when charged, their loss current is lower, typically under 100mA/100Ah, so an unregulated sytem will go to a higher voltage becaues of the lower current.
Stick to a wet one if you can't regulate the voltage!
 
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