Thread: current limiting lithium battery charging (by means of cable)

Originally Posted by drivesafe

Hi shady, that will work but be aware, to recharge that size battery with a 25 amp DC/DC device, if the battery is flat, you're be looking at at least 15 to 20 hours of drive time to get that battery up to 100% SoC.

Well I bit the bullet and bought a 300 ah lithium battery and swapped the 25 amp dc to dc to a 40 amp one which is connected to a solenoid activated by a dash switch which only works when the ignition is on. I plan to put the old 25 amp dc charger in my camper trailer and connect it to an Anderson plug which is connected to the vehicle battery through a VSR to charge 2 x 100 ah AGMs. In both cases the solar is through separate controllers. Will running both these at the same time worry the alternator, the vehicle is a 2016 BT 50 (unfortunately).

Originally Posted by sharmy

Well I bit the bullet and bought a 300 ah lithium battery and swapped the 25 amp dc to dc to a 40 amp one which is connected to a solenoid activated by a dash switch which only works when the ignition is on.).

If the vehicle does not have a smart alternator,and you have the solenoid wired as per your post,do you need the 40A DC to DC?

Originally Posted by scarry

If the vehicle does not have a smart alternator,and you have the solenoid wired as per your post,do you need the 40A DC to DC?

I think so. I am pretty sure the DC to DC charges the battery to 100% but the alternator on it's own doesn't.

Originally Posted by sharmy

I think so. I am pretty sure the DC to DC charges the battery to 100% but the alternator on it's own doesn't.

That was one of Redarcs selling points,but i am pretty sure Tim has other thoughts

Anyway,hopefully he will chime in.

Originally Posted by sharmy

I think so. I am pretty sure the DC to DC charges the battery to 100% but the alternator on it's own doesn't.

Hi sharmy ( I got your name right this time, sorry ) If you use very little out of the Lithium battery, then yes a DC/DC devices will fully charge them to 100%

If you use a lot of lithium battery capacity, then a direct alternator charge may not fully charge the lithium in the drive time, BUT if your alternator can not fully charge your lithium battery, then the DC/DC device will have no hope of getting any real charge at all into the lithium, in the same drive time.

a few weeks have gone by and some more results from testing are available so here it goes:

I have been testing the charge currents for a longer period and used the body as negative this time. The charge currents are not noticeably lower when using the vehicles body, as one normally would, for the negative/ground of the system. One could argue that the amount of metal is certainly enough but then again, most of it is only touching through spot welds. Still, I could push 100A with easy without negative side effects. I am not sure that I will use the body but it was a nice test nonetheless.

As of yet I did not find a cable to my liking with more smaller strands so I have not tested that yet. Seems I get enough current for now so the question is if I still need to test that.

Of more concern is my alternator. It is a 200A model and I have been in contact with the supplier and they rate it at 198c. During my last test run I had the engine running at 1750RPM for around 30 minutes so that everything heats up nicely and turned on everything I could find. 35A into the inverter, low and high beam, fans on full etc. During that time I would also see around 90A of charge current into the batteries. I reckon I would be running close to the rated output of the alternator at that point. The viscous fan would be in it's "low" mode ie slipping since it was simply not warm enough in the workshop but the engine did get up to temp very nicely.

I used a fluke FLIR camera to measure the temperatures and as expected the temperatures of the cabling and connectors were barely noticeable with regards to their surrounding. The air temperature being pushed from the radiator into the engine bay was around 80c and that is blown over the alternator for cooling. The alternator housing was fairly cold but that was to be expected; the emissivity of aluminium is quite a bit lower and the FLIR was not set for that. The parts of the windings that were visible through the cooling slits though measured really high. certainly above 198c though I could not find the emissivity value for enameld copper wire. I used copper but I might need to switch to enamel. Still, down bush or worse the simo that under bonnet temperature will certainly be a bit higher so I find myself living dangerously close to the temperature limit as advertised by the supplier.

Still, I did notice that after some time (and a worrying whiff) that the output voltage dropped so the regulator did notice the high temps and started limiting the output current accordingly.

The adventure continues!
Cheers,
-P

Hi Prelude, just a word of caution.

Be careful when producing high current outputs from the alternator while idling.

Even when moving slowly in traffic, massive amounts of air flows over and through the alternator.

When idling and stationary, the thematic fan is controlled by the heat of the radiator and gives very little cooling for the alternator.

The high currents you are producing while testing with the alternator. for such long periods of time and not getting a decent air flow over the alternators casing could easily lead to an alternator failure.

Originally Posted by drivesafe

Hi sharmy ( I got your name right this time, sorry ) If you use very little out of the Lithium battery, then yes a DC/DC devices will fully charge them to 100%

If you use a lot of lithium battery capacity, then a direct alternator charge may not fully charge the lithium in the drive time, BUT if your alternator can not fully charge your lithium battery, then the DC/DC device will have no hope of getting any real charge at all into the lithium, in the same drive time.

I should be right. The back of the ute with a 400 watt solar panel on the roof sits out in the sun most of the day and powers 2 engel fridges. The dc charger is for when I travel long distances.