Sawtooth charging from alternator?

I would check for damaged wiring or loose connections.
Assuming you have two 100ah in parallel, I would charge and test them separately.

I would seperate the two batteries. Check voltage and try charging individually.

How are the batteries charged, do you have a DCDC for instance?

More info as per the charging setup would be good?

What is the make and model of the tow vehicle?

Joe,
Is this the first time you have created these conditions or have you had the batteries this low before and the system has operated satisfactorily, in other words, can you confirm the system has ever worked and this is a recent development?
My guess is it is an equipment incompatibility or failure problem or recent modification. The pattern is too uniform for it to be a wiring fault that has developed.

Kinda looks like a Voltage Sensing Relay like a Redarc SBI or similar cutting in and killing the feed to the camper trailer when the batteries charge and pull down the vehicle system voltage (via whatever mechanism they are hooked up to).. or a DCDC charger itself determining the vehicle is not running just from the voltage level and ceasing charge - when in reality there's a Smart (dumb) Alternator in place..

As others have noted, more information required about setup.

G'day Joe.
Looks to me to be the action of a DC DC Charger going flat out, and reducing it's output because of internally generated heat and therefore almost stopping the charge delivery UNTIL it cools below a set internal threshold, ONLY to be TAXED again as straight away the heat build up again reduces output to SAVE the DC DC UNIT itself. Is the DCDC unit able to be cooled? Does it have an internal fan? is there copious amounts of COOL passing air so it CAN DO IT's JOB? if hidden away inside a caravan bin or cupboard there is almost NO airflow to cool a DC DC unit. Even having a DC DC mounted horizontally and NOT VERTICALLY can make a big difference to the ability of the airflow not being able to travel/pass upwards and carry heat away. Ok under light duty, but high current , maybe NOT!

While the Max load of deep discharged batteries is being applied to the DCDC , it will continue to do that.

You did say alternator charged but didn't specify if ALT to BATTERIES DIRECT or through a DCDC UNIT.

If the alt is used directly to the batteries, THAT can cause the alt to fail through CONSTANT HIGH AMP DELIVERY which they are not designed for, ie LONG TERM Duration of plenty near MAX amps.

I’ve hooked up a DCDC charger in my boat, to charge an auxiliary lithium battery. The boat motor only puts out 35 amps maximum. I was finding the charger was cutting it and out like this (sawtooth) and I assumed it was because charging the lithium was drawing power too quickly from the battery it was charging from, and that battery voltage dropped too much. I made some changes that fixed it.
What DCDC charger do you use and does your car have a smart alternator? I am no expert on electrics but I’m guessing your charger might be trying to charge the flat batteries at full capacity of 40 amps but is then sensing a voltage drop from the main battery, so stops. Then the main battery voltage comes up and the cycle starts again. I’d be looking the inlet voltage to the charger and see what it is reading at.

Thanks for the input everyone!

We have a 2021 Prado, can't comment on if it is a smart alternator.

Alternator is going through a projecta BMS before going into the batteries.

Perhaps the cutting in and out from flat was causing it. I'll hook it up to the car this morning and see what happens. I will report back.

You wouldn't happen to.have an auto reset circuit breaker in the feed from the original charging system would you?
Can you hear a ticking noise every 30 seconds or so?

Morning All,

We stopped by an auto electrician yesterday. He replaced the ends of the Anderson plug, checked the ground and wires. That didn't help.

He believes that there was a voltage drop when the DCDC charger kicked in and that caused the charged to go off. Suggested there may be voltage drop from front to end.

It then cycles back on and repeat.

He then looked for an 'on' wire in the projecta management system to power on when there is current instead of using a change in voltage to trigger. We're hoping that this solves the problem. I'll report back.

Joe,

Where is the smart shunt installed, ie before the charger or after the charger?

Can you post up the corresponding voltage graph?

As Loddo48 wrote the Prado 150 doesn't have a smart alternator and you shouldn't
need the ignition trigger under normal circumstances.

Another update as the plot thickens...
1) on a friend's car we get stable charge through the alternator. (Image FA).
2) since the adjustments by the auto elec we get a better charge on the battery with OUT alternator...but it still drops out . (Image A)
3) when charging with a solar blankets - with direct sun, I heard the DC DC drop out periodically with a slight whine sound. (Image S)

I can't work out where the problem lies as it seems that the issue is present with both solar and alternator, however, it appears with solar as well?! Or does solar drop out like this normally?

To me, it looks like a clear case of the cable between the Main Battery and the input of the DC DC charger is too long, or too thin, or both.


If it's less than a meter between the charger and the battery use at least 6B&S, if it's any further use 4 or 3 B&S. On both positive and negative sides