Another battery question.

[Clydewanderer]

Although non boaty im seeking advice.
Twice in last 2 wks i had to jump start my car, a vauxhall insignia 2.0cdti. I checked batt voltage before starting this morning using a good quality multimeter, got a reading of 12.11v then started car and ran without loading (lights, etc off) it gave a reading of 14.7v. I then took it to local auto factors who carried out a load test which quickly droped into red. I then started engine and fired up all loads, (lights, fan, wipers sterio etc) and the guy said alt was putting out 15volts claiming it was too high. That contradicts what my meter said 15 mins earlier. I feel he was trying to sell a batt and an alternator.
Should i do a volt test myself with all loads on and what should i expect to see?
I checked batt levels before volt test tday and all levels were ok which makes me think if alt was over charging would batt level not be down on at least one cell due to cooking?
Cw

 

[RichardS]

I would hope to see around 14.8V so your own reading was probably correct. I would just buy a battery and then test the output with a good battery but even if it was 15V I wouldn't be too concerned as car batteries very rarely get fully charged anyway.

Richard

 

[Bobc]

Just buy a new battery. That's what the problem will be.

 

[drakes drum]

Gassing starts at around 14.5 to 14.8v depending on temperature so in an ideal world your alternator regulator ought to keep the max voltage around there. If it had failed the voltage would be far higher or far lower so the regulator is working. Ignore the garage man.

Your starter motor is likely to be around 1000 watts and so draw around 100 amps at 12v. Likely your battery is failing to deliver that and the voltage will be dropping badly so the starter motor wont develop its power output.

Best thing to do is to connect another fully charged battery in parallel with jumper cables. If that cures the starting then the problem is your cars battery. Thats the most likely cause anyway.

 

[Clydewanderer]

The auto shop guy done the load test on the batt first which very quickly drew the batt well down into red on on his load tester then asked me to start engine and turn all loads. I would have expected the alternator to put out max voltsat that stage anyway.
The car did start imeadiatly when jumped from another car using very skimpy leads yestarday.
Think a visit to a battery shop is on the cards.
Thanks all.
Cw

 

[VicS]

The auto shop guy done the load test on the batt first which very quickly drew the batt well down into red on on his load tester then asked me to start engine and turn all loads. I would have expected the alternator to put out max voltsat that stage anyway.
The car did start imeadiatly when jumped from another car using very skimpy leads yestarday.
Think a visit to a battery shop is on the cards.
Thanks all.
Cw

https://www.tayna.co.uk/

But I think I would try bench charging it first. Also recheck and monitor the alternator output volts.

 

[savageseadog]

Measuring voltage is not as straightforward as the OP thinks. Some digital multimeters wan't always give you an accurate reading due to noise and AC superimposed on the DC. It will also depend on where measured, right on the battery or on the alternator.

 

[Sailingsaves]

But keep the 'duff' battery (1 - just incase alternator is the problem and (2 - because you could possibly rejuvenate it) if you are inclined.

My old car had a duff alternator - new battery would often drop to 12.2 V, but still start car. Alternator would often kick in 17 Volts !!! (I had rigged an onboard voltmeter) and I just turned on power drawing devices to drop voltage to 14.5volts. (Car was not worth even a second hand replacement alternator).

I simply plugged the car into a charger every night and swapped out the battery now and again to give it a 'clever' charge (borrowed ctek charger) to keep battery in good condition.

Lots of threads on here about rejuvenating batteries.

Even if it is duff, some nutters like me stick said batteries in garden with a cheap solar panel and light up lots of garden lights, 12volt pumps from water butts etc

 

[Davegriff]

Even if it is duff, some nutters like me stick said batteries in garden with a cheap solar panel and light up lots of garden lights, 12volt pumps from water butts etc

+1. I always keep my latest "duff" battery in the workshop, continually connected to a tiny "trickle" charge solar panel. Very handy when testing various bits of equipment - surprising how even 150ma solar panel will keep a 12v lead acid battery going for years if its not used much.