Could I ask for some advice on checking charging systems. Today I tried to help my daughter with a problem with her charging system (Honda HRV battery flat or maybe something more involved). 6 month old battery.

Anyway, I did what I've always done - checked battery voltage before doing anything else (12.27v). Car wouldn't start, so connected jump leads to the Focus, still wouldn't start which I thought strange. Left the Focus running for about 15 minutes with the jump leads still connected and her car then started immediately. Charging voltage showed ok at 14.2 volts 

Going back to the Focus, I removed the jump leads and checked the voltage on my own battery. Was surprised to get a reading of only 12.57 volts with the engine still idling. Test leads positioned on the actual positive battery terminal and the chassis earth.

I just don't get. Am I doing or have done something wrong? Never had a problem doing these checks before. Not had a problem starting the Focus.

 

Update. Drove the Focus home, tested the car again and it is now charging at just over 14v which is what I expected.

Did exactly the same check as earlier.

In contrast, I've now had a text from my daughter to say that her car wouldn't start again only 20 minutes after I left - despite the fact I had let it idle for at least half an hour at a measured 14.4v.

Totally confused.

35 minutes ago, RayC333 said:

Could I ask for some advice on checking charging systems. Today I tried to help my daughter with a problem with her charging system (Honda HRV battery flat or maybe something more involved). 6 month old battery.

Anyway, I did what I've always done - checked battery voltage before doing anything else (12.27v). Car wouldn't start, so connected jump leads to the Focus, still wouldn't start which I thought strange. Left the Focus running for about 15 minutes with the jump leads still connected and her car then started immediately. Charging voltage showed ok at 14.2 volts 

Going back to the Focus, I removed the jump leads and checked the voltage on my own battery. Was surprised to get a reading of only 12.57 volts with the engine still idling. Test leads positioned on the actual positive battery terminal and the chassis earth.

I just don't get. Am I doing or have done something wrong? Never had a problem doing these checks before. Not had a problem starting the Focus.

 

I don't know for sure and don't know if the Mk3 is the same as the Mk4. 

If it is then when the PCM/BCM decide to change the Charging Regime from Conventional Charging to one of the other  ones, Battery Regeneration I think, then the Alternator output goes to zero, which means it discharges when driving and I think at Idle also.  The only time it charges and it does so aggressively is when on the overrun when driving.

I've never tried checking the battery voltage the way you have, but with your negative meter lead on the chassis ground it could well be that the BMS shunt resistor in the battery negative lead is affecting the measurements.

Try doing it again with the meter directly across the battery terminals.

EDIT:

Just realised that yours is a Mk3 and not a Mk4 so most probably it does not have a BMS monitor in the battery negative lead.

Mk3 Focus uses variable rate charging.

If the battery is at least 80% charged, you're not using any electrical consumers, and the engine is idling, the alternator is effectively switched off to reduce fuel use and emissions.

Thanks guys, looks like I've been fooled by the car's electronics.

All seems normal now, for the Focus anyway.

19 minutes ago, TomsFocus said:

Mk3 Focus uses variable rate charging.

If the battery is at least 80% charged, you're not using any electrical consumers, and the engine is idling, the alternator is effectively switched off to reduce fuel use and emissions.

My battery was (and is still reading) 12.57v when left for a while. 

When I was doing the jump starting I turned off everything I could.

 

17 minutes ago, unofix said:

EDIT:

Just realised that yours is a Mk3 and not a Mk4 so most probably it does not have a BMS monitor in the battery negative lead.

Does have one Unofix. Only reason I used the chassis earth point is that on the MK3 Focus the neg terminal is in an awkward position.

8 minutes ago, RayC333 said:

My battery was (and is still reading) 12.57v when left for a while. 

When I was doing the jump starting I turned off everything I could.

You'll be upsetting the Mk4 owners with a resting voltage like that!  

I'm not sure of the exact figure that the PCM looks for but it'll be around 12.6-12.8v for 100% SOC on a 12v EFB battery.

Mk3 charging system is very similar to Mk4.  Both are variable and try to charge on over-run as much as possible.  Mk4 SOC cut-off seems to be set lower (Mk3 may well be nearer 90% than 80%).  I'm not sure whether the Mk3 also includes a regeneration cycle as Tizer mentions above.

No doubt my daughter's Honda HRV has a similarly complex charging system and/or won't start under certain conditions.

She bought the Honda (after offloading the Focus - with all its problems to me) for more reliability. Either she is very unlucky with cars or they are no better (2016 car, low mileage. Satnav packed up - Honda can't fix it, start button failed - replaced under goodwill, now starting issues). 

5 minutes ago, RayC333 said:

No doubt my daughter's Honda HRV has a similarly complex charging system and/or won't start under certain conditions.

She bought the Honda (after offloading the Focus - with all its problems to me) for more reliability. Either she is very unlucky with cars or they are no better (2016 car, low mileage. Satnav packed up - Honda can't fix it, start button failed - replaced under goodwill, now starting issues). 

A 2016 car from any manufacturer will have to meet Euro 6 emissions regulations.  So I would assume that they also use variable charging as part of the emissions reduction package.

I don't have any experience with modern Hondas though.  I briefly worked at a Honda dealership nearly 20 years ago.  Barely saw any faults at all back then.  It was about 95% servicing & tyres.

54 minutes ago, TomsFocus said:

Mk4 SOC cut-off seems to be set lower (Mk3 may well be nearer 90% than 80%).

When you set your Mk4 to a SOC of 95% then things improve 😀