Voltage drop curve of lead acid batterry with increasing discharge

C

Chanquete

Well-Known Member
Joined
18 Jan 2013
Messages
202
Location
Buenos Aires, Argentina
Visit site
Hello
During the transient connection of the glow plugs the voltage of my 12 V lead acid battery drops to 10 Volts.
After the start of the engine the voltage rerturns to normal values.
Is this voltage drop exessive?
Shall I change the battery?
Thanks in advance for any orientation
 
Once the engine is started the power will all come from the alternator at which point the battery value can't be measured, you will only then see what voltage the alternator is sending to the system including the battery, so it sounds like your alternator is ok.

So I assume this drop to 10v is before the starter is operated. This does seem low, but premature yet to simply condemn the battery.

1 Repeat the measurement with an accurate VM at the battery posts - is it the same ? If higher then likely dirty or loose connection/s somewhere, and do not overlook the negative return when searching.

2 How well does the engine spin on the starter - is it sluggish ?

3 What is the precise battery level before the glow plugs are operated (other loads off) ?

12.8 v good
12.6 v part discharged
12.4 v quite well discharged
12.2 v almost flat
12 v flat

Many panel meters are not calibrated so worth checking this at the panel meter connections with a decent VM. Moving needle meters do not show the precise difference between 12 & 12.8 v accurately enough, so a digital display to at least one and ideally two decimal points is far better.

4 How long since the battery was on a separate charger ?

5 What other loads have been on since last on charge ?

6 If an accessible lead acid type have you checked the SG of the electrolyte with a hydrometer ?

Or does it have a magic eye - is this clearly green ?

7 How old is the battery & is it the correct size for the engine -CCA & AH ?
 
Last edited:
It’s not necessarily excessive as far as I understand things. But we’ve got some real experts on lead acid batteries and their characteristics on these forums and I expect one will be along soon.

Edit. Superheat asks some pertinent questions in the meantime. (And my post crossed with his)
 
Last edited:
As Superheat correctly says, the increase in voltage reading is caused by the alternator output, so has to be disregarded. Take the following voltage readings from the battery terminals ;

Before operating the glow plugs
Whilst operating the glow plugs
After operating the glow plugs, without starting the engine
10 minutes later

What are the readings ?
 
Mine drops to around 10.6 volts.I think with out looking it up that the glowplugs are rated to around this figure.
 
Chanquete said:
Hello
During the transient connection of the glow plugs the voltage of my 12 V lead acid battery drops to 10 Volts.
After the start of the engine the voltage rerturns to normal values.
Is this voltage drop exessive?
Shall I change the battery?
Thanks in advance for any orientation
Click to expand...


If the engine ( MD2030B I believe) cranks normally and starts after the ( 10 second ?? ) preheating period then there is no immediate need to change the battery.

I assume you are reading the 10 volts directly from the battery terminals with a decent voltmeter or multimeter. If so 10 volts is rather lower than I would have expected. Therefore budget for a new battery in the near future.

VP recommend a 70Ah battery but do not specify the more important CCA rating.
 
Last edited:
PaulRainbow said:
As Superheat correctly says, the increase in voltage reading is caused by the alternator output, so has to be disregarded. Take the following voltage readings from the battery terminals ;

Before operating the glow plugs
Whilst operating the glow plugs
After operating the glow plugs, without starting the engine
10 minutes later

What are the readings ?
Click to expand...

Thanks for the advice Paul!
Here are the answers

Before operating the glow plugs 12Volts

Whilst operating the glow plugs 10 Volts

After operating the glow plugs, without starting the engine 12 Volts

The engine starts immediately in much less than a second.
 
I would start by ensuring the battery really is fully charged.
24hrs on a good mains charger.
 
When I operate my bow thruster my domestic bank of batteries 12 x 105 Ah drop to about 10 Volts but return to near fully charged voltage when stopped. My bow thruster draws something like 600 Amps

I put this down to internal resistance within the batteries.

Any heavy current draw will drop the external voltage of a lead acid battery and other types due to internal resistance.
 
Chanquete said:
Thanks for the advice Paul!
Here are the answers

Before operating the glow plugs 12Volts

Whilst operating the glow plugs 10 Volts

After operating the glow plugs, without starting the engine 12 Volts

The engine starts immediately in much less than a second.
Click to expand...

At 12v the battery is effectively flat, at about 40% although it starts the engine. I'd look into why that is happening, is the battery usually on shore power charge, solar charge, or what ?
 
Chanquete said:
Before operating the glow plugs 12Volts

Whilst operating the glow plugs 10 Volts

After operating the glow plugs, without starting the engine 12 Volts

The engine starts immediately in much less than a second.
Click to expand...

If the battery is only reading 12 volts with no load and without having been on load for several hours previously it is almost flat ... at least down to about 25% of fully charged

Maybe as, lw395 suggests it merely needs a spell on a good automatic, multistage charger, with an appropriate maximum amps output, to recharge it fully

HOWEVER

The fact that the engine cranks and starts easily suggests that the redaings may not be accurate, Are they readings taken directly from from the battery terminals with a decent reliable meter
 
The fact that the OP wrote "12 Volts" rather than 12.0 makes me wonder if he's using a simple analogue voltmeter, which is usually fairly inaccurate. Readings with a decent multimeter might be more relevant.
 
pvb said:
The fact that the OP wrote "12 Volts" rather than 12.0 makes me wonder if he's using a simple analogue voltmeter, which is usually fairly inaccurate. Readings with a decent multimeter might be more relevant.
Click to expand...

Good point
Although an analog meter need not be inaccurate, but if the readings are from a crude panel meter they may well be.

The fact that the engine cranks and starts easily suggests they are inaccurate and the OP may be trying to fix something that is not broken
 
Last edited:
The battery was full charged with an intelligent charger. The battery is 6 years old. The engine starts immediately in much less than a second.
I will check the volts with an electronic tester.
Thanks!
 
VicS said:
Good point
Although an analog meter need not be inaccurate, but if the readings are from a crude panel meter they may well be.

The fact that the engine cranks and starts easily suggests they are inaccurate and the OP may be trying to fix something that is not broken
Click to expand...

Thanks! . My Volt data comes from the instrument panel meter.
I will measure Volts again with an electronic voltmeter.
 
VicS said:
Good point
Although an analog meter need not be inaccurate, but if the readings are from a crude panel meter they may well be.

The fact that the engine cranks and starts easily suggests they are inaccurate and the OP may be trying to fix something that is not broken
Click to expand...

You may well be right, our panel analogue volt meter is out by 0.8 volts!
 
Note that cheapo electronic volt meters can be out of calibration. So worth a double check if readings seem wrong.

0.8v wrong for one of mine.
 
Most GPS chartplotters (if you have one) have built in voltmeter which can be displayed and much more accurate than panel volt gauge.

My eberspaker heater doesnt start up if less than 10.5v and I normally cant get it to start without running engine, and thats on a brand new battery. The hot plug load current drops the voltage much the same as engine glow plug. Battery gives about 12.5v to just under 13v under normal loads but rises to 13.5 to 14.1 if engine running. Thats on both of our boats.
 
I find this obsession with batteries and voltage in our typical boats strange. Ideally and usually the boat has a 2 battery system leaving redundancy. So with a back up you have much less to worry about than your car. I understand that keeping voltage up and minimising depth of discharge can enhance the life of the battery. However I reckon people worry too much. When a battery has reached the end of it's life it will be obvious. Don't worry till then. ol'will
 
You must log in or register to reply here.

Share this page

Top