Reading the battery voltage & alternator output

[freddie on Voyager @ Wychnor]

Hello, I have had a look around and can't find instructions for checking the voltage of batteries and the output of an alternator using a multimeter.

Please can someone tell mme the dos and donts. Cheers, Freddie.

[by'eck]

Set your multimeter to read DC volts. If its not a digital one (dial with needle) set it to read DC volts in the 0 - 25 volt range say but certainly above the highest voltage you will encounter (max 15 volts). Ensure black meter cable is plugged into common -ve & red lead into something which says +ve volts/ohms or something similar on multimeter.

 

With engine off, battery charger off, no load on batteries, place black negative lead of meter on common battery negative & red positive lead on the positive connection of battery(s). Note exact volts.

 

Start engine, run at fast idle say over 1000-1500rpm. Repeat battery volt measurement. It should be noticeably higher.

 

Note exact voltage then measure at alternator. Black meter lead on alternator case (or B- terminal), red on main B+ terminal of alt which should have thick red lead attached. If the wiring is good it should be almost identical to voltage on batteries assuming no diode splitter. Be careful not to let leads or anything else catch on drive belt. Repeat battery & alt voltage measurements in case the charge current has dropped due to batteries being fully charged.

 

You won't be able to measure charge current with your multimeter but the higher battery voltage with engine running is a good indication the alternator is charging batteries.

[AlanM6]

[Tony Brooks]

Hello, I have had a look around and can't find instructions for checking the voltage of batteries and the output of an alternator using a multimeter.

Please can someone tell mme the dos and donts. Cheers, Freddie.

 

 

Only use a Clamp type ammeter to measure the charging currant unless you have and are experienced in using high currant ammeters with with external shunts. If your alternator output is 70 amps or less then Durite make a 75-0-75 clamp type ammeter for about £25 (check Vehicle Wiring Products) but I understand higher range multimeters with DC (thats important NOT AC) ammeter clamps are available on Ebay.

 

You need to check the charging current when the battery concerned is in a low state of charge so for single alternator boats and domestic alternators on twin alternator boats do it after a night aboard at first start. For twin alternator boats' engine batteries you will probably have to flatten the engine battery by operating the stop and cranking the engine for several 30 second bursts.

 

Ensure the drive belt is tight and in good condition!

 

Fit meter (keep the Durite ones away from magnetic fields), start engine and immediately rev to at least 1500rpm. Quickly take reading because the current will start to drop almost straight away. If the reading is nearly the rated output then the charging currant is probably OK. The clamp goes around either battery cable or the main alternator output lead depending upon the situation.

 

Voltage is harder because at first start up it will be lower than the regulated volatge unless the battery concerned is well charged so I normally advise that charging voltage checks are carried out just before engine shutdown at the end of the day. You should really also check the voltdrop along the charging cables and for that you need a high current flow so that is done concurrently with the ammeter test.

 

Voltdrop:-

 

Set the meter to 20V DC (200V DC for 24 V boats). Connect between the alternator's main output terminal and the relevant battery positive. Rev engine to maximum current and hopefully you will find a volatge reading of less than 0.3 volt. Anything higher suggests a potential wiring or terminal problem whilst above about 0.7V suggests you have a split charge diode and unless the alternator is battery sensing you need to think about getting a DECENT split charge relay fitted.

 

Repeat for the negative (you may have to connect to the alternator case if it has no negative terminal) but this time the reading will normally be much lower.

 

Charging Voltage:-

 

At the end of the day clip the voltmeter across the battery terminals and rev the engine. The expected reading should be in your engine manual but as a rough guide:

 

Early alternators from the 70s & 80s - 13.8 to 14.2 volts

Other alternators more than (say) 8 years old - 14 to 14.4 volts.

Modern alternators - 14.2 to 14.8 volts but note the higher reading may cause problems for certain battery types so then will need constant level checking.

 

The lower voltages although correct may cause problems for the latest battery technology.

 

 

If you have a rev counter playing up, a glowing warning lamp or low current/volts connect the voltmeter between the alternator's main output terminal and the waring lamp terminal. Rev the engine and the reading should be well under 1 volt. If it more then the alternator is faulty.

 

 

Battery volatge readings are meaningless unless the battery has stood for several hours after being charged OR it has supplied some power for a while.

 

 

Did you not find http://www.tb-training.co.uk/MarineE08.html#CHARGING SYSTEM ALTERNATOR

Edited June 16, 2011 by Tony Brooks