question

To me it seems to be a connection issue of the AGM Battery to the Alternator and electric of the car.

Check the cabels. Its maybe just by bad luck and bad coincident that this behaves now like that.

In my case I had a connection next to the alternator that was completely corroded. And by bad chance same, did some modifications and one day later the issues. But obviously no causality...

Hi Chrichri and thank you for your answer.
First, I need to explain more precisely how the car mechanic has installed the booster.
1) booster put close to , but not on the battery (and the issues came with low outside temperature (about 5 to 10 °c) and cool motor (water temp. below 80°c),

2) connected with adequate cables (AWG6, fine copper strands, eye crimp terminals), cables are less than 30 cm each,

3) no modification to the engine battery connection (in particular to the alternator),

4) booster directly connected to the battery screw system ( power bar directly plugged to the battery + lug) and to the chassis for the -).

5) 2 adequate car fuses (eye crimp connection) fuses, one on the plus cable booster-engine battery, one on the plus cable booster-auxiliary battery 80amp each (probably a little too much),

6) plus cable to the auxiliary battery : path below the car mat to the car boot (about 2,5 meters) .

7) minus cables :
- 2 cables from the booster to one dedicated chassis screw on the motor compartment (30cm), ( 2 because the booster version is the galvanic isolated one)

- and auxiliary battery minus cable plugged to one chassis screw in the car boot.

8) auxiliary battery is an AGM low discharge model (12V 95AH 850A) and main battery is the genuine reference for the car ((12V 70AH 760A)

Now about your hypothesis. As I said, there is no modification between the alternator and the battery, and things were going smoothly up to now without the booster. I can imagine that there could be some overcurrent problem, what is strange is that there seems to be no specific conditions for that : auxiliary battery already charged -at least 75%- very low power consumption at the charge side (50W) etc.

I probably need to deeply explore your hypothesis, but how? Is there a way to smooth potential current spikes coming from the booster , if any?

Hi, I would replace the fuse to the booster by a 20Amps fuse. The booster is a 18Amp you write. Maybe they recomend 25Amps but 80 is not good! If you get a short in the booster you will get in troubles with an 80 Amps fuse. Wit 20 or 25 Amps you can be sure that it will blow before the car stops. And then you know... Check once more the connection clamps to the main battery (Engine and alternator) They should be well placed to ensure a good contact. Check the minus conections very well. I would put one cable (DC DC input) to the same Minus screw from the main Battery and the other (after DC DC) to that you have choosen and to second battery etc.

I will note that the Peugeot 5008 series has a long history of electrical faults, going back to forum posts from 2012, so while I don't see that you specify your year or which hybrid system you have in the vehicle, I would strongly suggest having your mechanic deepscan for stored fault codes.

I understand (I think) that you don't have any active error messages, but that does not at all mean that there isn't a stored fault code in the vehicle's computer, and it would be worthwhile to find what that might be. Due to the complexity of the electrical system in those -and especially the newer vehicles- there is a very real possibility that special steps need to be taken to install an aftermarket electrical system in tandem, since the vehicle itself already has multiple interconnected electrical systems running.

Noting, in fact, that the very first page of the Peugeot 5008 user manual includes a specific warning about installing electrical accessories:

Hi Chrichri and Justin, Thank you for your comments and suggestions.
Related to Chrichri's comments: for sure I am going to recheck the connections (3rd time). The minus cables to to booster are screwed together at a chassis screw close to the main battery. One solution maybe (as suggested by one mechanic who debates against the other !...) is to put the booster in the rear boot and to have both minus cables of the booster and the auxiliary battery one attached together at a chassis screw in the rear boot, while the plus cable from the main battery is left as it is now (from the on-purpose screw which is part of the big metal bar connected to the main battery lug to the rear boot). Maybe the 2,5m cable could smooth a little the current spikes if any. As to the fuses, you are perfectly right and I will change that immediately. I still don't understand the reasoning of the mechanic who insisted for the overrated 80 Amp fuses. For sure I prefer a blowing fuse to an immobilized car! Connector clamps to alternator no checked yet, but never any trouble till now (12 years). I will check anyway. New main battery and positive clamp manipulated and checked many times since the mounting of the booster.

Related to Justin's comments.

- As to the car itself : 12 year old, but in perfect condition (I think ...) . Its a plain diesel model ( not a hybrid), EURO 5 standard, with stop & start, smart (?) alternator etc. Up to now, I have not checked the computer error messages (in particular from the main one i.e. BSI calculator). Fortunately I have a diagnostic tool (diagbox for peugeot), and I am going to see if there are messages left since the breakdowns (last breakdown was yesterday). Probably a record could exist if the BSI itself stopped the electric system after discovering something. If not, the power failure being instantaneous without warning, I doubt the BSI has time to record anything useful. Let's see.
- As to the warning message from Peugeot you posted above, you are right to mention it, but I don't know any car manufacturer who does not publish the same warning! And as you say , the more recent the cars are, the more sophisticated and unpredictable electric systems they embark. But this would lead to discouraging the use of charger boosters, which is not my goal!

- As to the record of 5008 electrical faults, I have not seen - for the moment- anything related to a sudden complete electric failure that could lead to a similar cause (rather light or fuse problems).
In my case, the alternator is the original one, with no problem at all till now,(steady 14,7v, no fluctuation, when the motor is on). New serpentine belt ( changed as recommended by the peugeot maintenance spec. after 12 years).

I am grateful for your suggestions, but still very annoyed: sporadic failure happening abruptly after the car has run smoothly during hundreds of kms, very dangerous situation when it happens. For sure no motorway travel for the moment with the system installed. And as I said, this is not a comfort auxiliary system, but a mandatory way to carry medicinal items which must remain at low temperature everytime.

I can completely understand your frustration due to the sporadic feilure and danger of the effect. In fact, an electronic to switch off completely will in anyway record a fault code. This will be overvoltage or undervoltage. If overvoltage then I would check if the booster puts energy in backward direction. This should not happen. In that case a simple diode between main battery and booster will completely avoid this and would be the most simple way to test it. If it is an undervoltage a heavy load is caused from the booster. This you will avoid with the 20 - 25 Amp fuse. It will blow and indicate that. In this case as well the booster has a fault because it should not cause loads like that.

If you have GND problems then I would suggest to use the galvanic separation of the booster as advantage. Split via the booster both circuits and if the GND is the same fine if not fine.

Hi Chrichri, Thanks again for your help.

1) I could extract the record from the diagnosis tool. However, as expected it is a little frustrating, because there was no power at the time of the registering . Few usable information : Vbattery 11.818v, Venergy accu (stop&start) 4.8v, motor stopped, only strange info about energy accu. temperature 97°C.

2) Yes I can try a power rectifier in series on the positive line (e.g. 70HF120, which is 70 A capable in direct mode). There is no information about the kind of protection installed in the booster.

3) the last point is the most interesting. I have moved the booster in the rear boot. Therefore I can manage a true galvanic isolation. The max steady power consumption of the frig is claimed to be 50W thus less than 5Amps, however there could be spikes (?) when the compressor starts.

The final point I don't know is the overcurrent when the booster starts feeding the auxiliary battery. I have tried to better understand how to set up parameters to minimize the current drain from the main battery (after all the power consumption from the frig is not so big and any medium duration travel (1 to 2 hours? ) should be largely enough to refill the auxiliary battery° So the strategy would be to set up the parameters accordingly. Reading the Victron spec. for the 12V/12V 18A booster (p 10 -13), one might think that the voltages thresholds should be raised and time delays made longer to ask less from the primary side, provided there is no overcurrent when restarting the auxiliary battery charging ( probably a key would be to find a way to smooth the auxiliary battery startup charge, but which way?) . This is only an hypothesis and here I think I need the advice of a specialist.

The difficulty and risk is that there is no way to know that it works (more than 1000km before it happened last time), but only to know that it does not work if the car stops without notice (unless the fuses do the job).