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12v battery charging

According to the Renault Twizy vehicle user manual the 12v battery is charged as follows:

12 volt battery
The second battery on your vehicle is
a 12V battery: this supplies the energy
required to operate vehicle equipment
(lights, washer/wipers, etc).
The 12 V battery is charged either:
– when the traction battery is charged;
– when the ignition is on;
– when the ignition is off and periodically,
provided there is enough
power in the traction battery.

After not using my Twizy for 9 days, with the PowerBoxBT left turned on (traction battery fully charged), my 12v battery is completely flat and when the key is turned nothing happens.

Theoretically the 100mA load of the PowerBoxBT is more than sufficient to discharge the 14Ah 12v battery (0.1A x 24 x 9 = 21.6Ah).

I expected the 12v battery to keep itself topped up from the traction battery but this seems not to be the case.

I plugged the Twizy into the mains and initially nothing happened but after a few minutes it started to charge the traction battery, which was still showing 98%. I assume this started when the 12v battery had sufficient charge.

It seems my 12v battery charges when the traction battery is charged and when the ignition is on but not periodically when the ignition is off.

This isn’t a big problem as all I need to do is remember to turn off the PowerBoxBT.

I’m now wondering whether this is a fault or just normal? After reading many posts about the 12v battery I’m inclined to think the 12v battery doesn’t keep itself topped up when parked as the manual suggests.

Further to the above I have today done a bit of measuring with my multimeter.

As stated in the manual the 12v battery is charged when the traction battery is charging.

It also seems the 12v battery continues to charge when the traction battery is full (fans and screen off) but the mains is still plugged in.

With the PowerBoxBT turned off the standby current of my Twizy is ~12mA, which will theoretically drain the 12v 14Ah battery in ~48 days.

With the PowerBoxBT left on the standby current of my Twizy is ~205mA so the 12 battery will be flat in ~3 days.

The conclusion is I need to get into the habit of turning off the PowerBoxBT.

Further to the above I’ve since found this in my service manual.

From 15/08/2014, a system to maintain the charge of the auxiliary battery will be fitted to the vehicle.
Part numbers for the first generations of charger not taking the charge function of the 12 V battery into account during the vehicle sleep phases: 296054684R, 296055169R, 296052118R, 296051612R, 296058331R, 296050008R.
To prevent discharge of the auxiliary battery when the charging plug is not connected, a charging program for this battery has been implemented. Without any external visual sign, 35 minutes after switching off the + after ignition feed, the charger activates and charges the auxiliary battery if necessary. The charger uses the voltage of the traction battery to recharge the auxiliary battery. If the charge percentage of the traction battery is less than 15%, this recharge function is not authorised or is cancelled.

My Twizy was registered on 16/02/15 so may have been built prior to this modification.

I need to check the part number of my charger. I hope it’s not an early one as I specifically went for a later model for the upgraded charger but not too late for the PowerBox.

Due to a further 12v battery charging issue I’ve removed and dismantled my charger today to check for any water ingress.

All seems ok but it looks like my 2015 registered Twizy is actually fitted with a 2012 charger!

Further to the above I have since replaced my charger due to it failing to charge the 12v battery under all conditions other than driving fast!

To conclude, I have done some tests with my new charger fitted.

When the ‘ignition’ is on, my Twizy is charging or charging is complete but my Twizy is still plugged into the mains the 12v battery voltage is maintained at around 14.4v.

This is the interesting part. When my Twizy is parked the voltage measured at the 12v battery never falls below 13.2v, periodically jumping up to 14.5v for around 4 minutes. After my Twizy was parked for a couple of days, this was happening about every 40 minutes - see chart below.

This seems a bit excessive, especially as a relay is pulled in every 40 minutes!

Maybe this would be different if I fitted another new 12v battery, now my charging issues are resolved?

I doubt it. The voltage on the 12v battery isn’t dropping very fast.

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I think I’ll leave it for now and see how it goes.


I carried out a further test today, measuring the 12v battery standby current.

During the period the voltage is low there is ~12mA being drawn from the 12v battery.

During the 4 to 5 minutes the voltage is high, the current into the battery is as shown below (samples every 15 seconds).


I’ve done some further calculations and this is negligible compared to the 6.1kW traction battery at ~0.006kWh per day (providing my maths is correct).

I’ve put it all back together and now intend to do less testing and more driving!


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I’m glad I found this thread - so my powerbox could explain this…

I will unplug the box and refit the battery - freshly charged by the sunshine today - and see if it still drains.
I assume that even though my car is a late 2014 the charger is the first gen - and.so will need replacing at some point.
There is a new EV garage opened up near me - so he can have an investigate into the old charger once it is removed and see if he can make it work like the new one does. Might be a cheaper and easier solution for the original chargers.

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I confirmed my fears that the traction battery isn’t charging my accessory battery with my gen 2 charger which was fitted 4 years ago.

Voltage is static just over 12.8 Volts. Traction battery is 75%.
Guessing the relay that gets pulled in every 40 minutes has burnt out :grin:

Is that a relay internal to the battery or the charger. If it’s the charger then surely it’s a case of locating and replacing.

I need to go looking :grinning:

Are you sure it’s a second generation charger?

Also, it might be worth giving it a little longer.

I can’t help but think my 12v battery charging is excessive as it starts as high as 13.2v!

Renault changed it about four years ago , before I had the problem charging with high mains voltage, it went in to the garage about 3 times before the charger finally failed

Had to wait four weeks for a replacement to come from France and get fitted.

Depending on the state of the accessory battery I have seen 13.2V to 14.2V from the traction battery with the ignition on.

I will take off the front steel plate and see if I can see a label on the charger.

If you get it on a ramp make sure to photo - I want to investigate mine. I am sure I have a gen1 charger - it pulls over 2kw from the plug - as measured in my smart plug.
But my ctek shows that it manages to top up my 12v battery when charging. I thought only the gen2 did this. It got it to 12.95v.

Isnt 14.1v the magic number?

I’d be interested in having an EV specialist see if he can refurbish the charger. We have a new garage near me who specialises in air conditioning EV taxis/ Prius etc - he though the Twizy should be a simple fix?!?!

@Kwh-hunter You are not too far from @grumpy-b who is an expert with EV’s and Charging units.

The Gen 1 charger does charge the 12V during the charge process. But not when the Twizy is turned off and parked up for the night. I understand the Gen 2 charger tops up the 12V even when the Twizy is turned off and parked up.

Time how long after reaching 100% before the fans switch off. Gen 1 over 20 mins up to 7 hours Gen 2 only a few minutes. Not sure the Gen 2 charges as well or balances as well as the Gen 2. May range dropped after the swap.

Oh great - that could be an option then.

My new 12v exide drained totally in 2 days - so I assume the traction battery didnt try to keep it charges - unless it is a dodgy 12v - or the powerbox consumes more than the traction can put back in.

I have some ramps - so will put it up to have an investigate - and the mandatory clean. Seeing the rusty frame on another thread - probably adding some hammerite or waxoyl to anything that looks exposed aswell. Having done the brakes I want to do the hubs and exposed suspension too.

Is the fan visible once this front plate comes off?
My fan seems to switch off quickly once 100% is reached - but it maybe because my battery is already balanced well?

The fan is very small and is in the front of the charger under the drivers foot. Got to via the floor access panel. It isn’t easy to see as it is covered. See the charger change thread for pictures.

The Powerbox does flatten the battery in two days, but perhaps not the Powerbox alone just that it keeps part of the CAN bus powered.

Under the Twizy the charger has a date of 2013 so it has been changed as my Twizy was registered April 2012.

Need to check the part number to see if it’s a gen 2 charger :thinking:

This link has pictures you need to compare versions. Also how to change.