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  1. #1
    Join Date
    Jan 2013
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    Buenos Aires, Argentina
    Posts
    131

    Default Voltage drop curve of lead acid batterry with increasing discharge

    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

  2. #2
    Join Date
    Jan 2012
    Location
    South Coast
    Posts
    5,204

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    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 by superheat6k; 14-09-19 at 06:35.
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  3. #3
    Join Date
    Jul 2002
    Location
    Farnham, Surrey
    Posts
    21,425

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    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 by john_morris_uk; 14-09-19 at 06:33.
    Semper aliud

  4. #4
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    8,060

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    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 ?
    Rainbow Marine.
    www.rainbowmarine.co.uk

  5. #5
    Join Date
    May 2009
    Location
    Wirral
    Posts
    694

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    Mine drops to around 10.6 volts.I think with out looking it up that the glowplugs are rated to around this figure.
    Leisure Owners Association

  6. #6
    Join Date
    Jul 2002
    Location
    Home: Kent. Boat: Chichester
    Posts
    43,619

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    Quote Originally Posted by Chanquete View Post
    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

    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 by VicS; 14-09-19 at 17:27.

  7. #7
    Join Date
    Jan 2013
    Location
    Buenos Aires, Argentina
    Posts
    131

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    Quote Originally Posted by PaulRainbow View Post
    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 ?
    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.

  8. #8
    Join Date
    May 2007
    Posts
    38,379

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    I would start by ensuring the battery really is fully charged.
    24hrs on a good mains charger.

  9. #9
    Join Date
    Nov 2001
    Location
    Me: Johannesburg South Africa Yacht: Richards Bay East Coast Africa
    Posts
    7,136

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    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.
    Life is too short not to have a sea view
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  10. #10
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    8,060

    Default Re: Voltage drop curve of lead acid batterry with increasing discharge

    Quote Originally Posted by Chanquete View Post
    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.
    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 ?
    Rainbow Marine.
    www.rainbowmarine.co.uk

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