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

@drivesafe thanks for the tip. It feels like the belt driven fan does produce a lot of airflow but perhaps it is insufficient indeed. I will place a large fan on top during testing to simulate airflow as much as possible.

@sharmy you SHOULD be alright since by all accounts most alternators will drop the voltage when they are overloaded and thus the input voltage will drop below the DC/DC chargers cutoff point. This does mean your starter battery will suffer a bit though. In my case, my DC-DC is also my solar input so it takes from the panels what it can and then moves on to the alternator input. Besides, 65A is not THAT much for an alternator, depending on what else you run at the same time. Note that the engel fridge will run from the aux batteries which are powered by the DC-DC, in other words, your batteries charge slower but you are not pulling more from the alternator. Everything that is connected to the AUX remains within the 40/25A limits of the DC-DC's.

Cheers,
-P

Finally, another update!

So I got around to buying a metre of boom-car cable with finer strands. The current it can push through seems to indeed be higher than similar high-voltage course stranded wire. ie. the voltage drop is lower, something to keep in mind!

With that shiny new piece of cable I redid my test setup but in a way that I could test with the bonnet closed and driving. The results were... less impressive. I did not even get to the driving and testing part... Lets get into the setup first:

First I installed a Victron shunt between the alternator and the car battery. In order to do so I had to modify the mounting of the alternator in such a way that it was electrically isolated from the aluminium bracket holding it onto the engine. I realize that it would reduce cooling ever so slightly due to the loss of thermal mass but I figured that would be negligible. Victron shunts are setup such that they only work in the negative lead so I had no other option in this case.

The second thing I did was install a second earth strap between the battery terminal, where the shunt/alternator also terminates, to the chassis/body to increase the total diamater of the ground cable/straps and the contact area to the metal to reduce losses / increase current carrying capacity. The standard setup is barely up to the job with the default alternator so I simply doubled it.

Thirdly I installed the lithium batteries, my CTEK setup and inverter in the back of the car in stead of on a test bench. I ran both 5m lengths of 50mm2 from the positive terminal into the car to the CTEK input terminals. From the CTEK output terminals I ran both short runs of 50mm2 and I hooked the bmv-700 shunt up to a large flat piece of copper strip that acted as busbar between the negative terminals of the batteries and the shunt. From the shunt I used my newly acquired test lead of 70mm2 and hooked that up to the body work of the car, to the passenger side mount point of the seatbelt to be exact.

I also used the rear mount point for the seatbelts to run a length of 25mm2 as ground for my inverter. The positive lead of the inverter was hooked up to the switched output of the Smartpass.

In order to load the alternator up even further I hooked my 600watt 12v boiler up to the car battery terminals running two lengths of 35mm2 that I had laying around, one short one long.

After checking everything over I discharged the lithium batteries to around 50% by running a 500 watt load on the inverter for a couple of hours. I kept the inverter loaded and running to push the load on the alternator as far as I could go. I ran high beams, the blowers at full and switched on anything I could.

My observations:

• at idle the current was around 120A, makes sense most alternators can not do full output at idle.
• increasing the engine RPM to 1500 I got to about 180-188A. This was as much as I could get, I had no more things to switch on to load up the alternator further
• the alternator voltage dropped to only 13.89 volts at the terminals. I reckon the regulator was not yet pulling back nor was it overwhelmed (yet)
• with the inverter off I would push close to 90A into the lithium batteries
• the D250SE would be doing around 20A of that 90A total
• It seems the DC-DC always running max output, see below
• with the inverter on the charge current into the batteries would drop to 50A
• I seem to "loose" 5Amps somewhere since the inverter on it's own pulls around 35A. Probably has to do with added cable losses and thus voltage drop.
• The belt driven fan was fully engaged since it only disengages at higher RPM. ie full cooling was applied.
• the alternator was getting way too hot

Most of the above requires no explanation. The most worrying thing is the alternator. I measured the temperature between the windings with my thermapen instant meat temperature thingy and it kept climbing up to the point I stopped the test at around 205c (the manufacturer stated that 198c was the design limit). Since the belt driven fan was running at around 1500RPM the fan that I placed on top of it barely made a difference. I think we can safely conclude that the alternator I purchased MAY be able to put out 200A but it is certainly not capable of doing so "full duty-cycle". I do not claim that no alternator can do that and that drivesafe and others are incorrect, I tend to believe that factory systems are most likely capable of doing full output without going into meltdown, but I do claim that this particular unit is a bit more sales pitch. As we tend to say; a lotta show and no go. Mind you I am aware that even in slow traffic there would be more airflow. Perhaps I am looking for trouble by creating a very specific failure case but... the under bonnet temperatures would certainly be very high when driving in a hot climate, in low range at low engine speed (say the 1500RPM mentioned above). My test was done with a cold engine, heating up as the test ran along in 20c ambient temps with the bonnet open AND an added fan to increase all the chances of cooling/airflow.

Imo the alternator has either: no temperature compensation in the voltage regulator, or the regulator and diodes are rated for the current and they just don't get hot enough and it's just that the stator windings can't handle the current, or: the cooling of the alternator is insufficient. I tried to provide as much airflow as I could possibly manage but these tests have made me a bit weary to test it whilst driving since I can't properly monitor the temps then. Since the housing is exactly the same as the standard 120A unit perhaps it is simply not capable of handling more thermal load or airflow for cooling.

In any case, for me this has meant contacting the seller and seeing what they have to say but also ahead of that searching for another alternative. I found one and shall link it here for posterity, though I should do that in the topic about high output range rover alternators but my search skills seem to fail me since I can't find it Alternators | 300 Series Alternators | Land Rover Defender | Discovery | Ranger Rover 320A Alternator they have even higher output units for those of us that need it... but I'll stick with this one since it should be sufficient and fits in the standard location.

Regarding DC-DC charging. In setups with a smartrelay or smartpass in my case and a DC-DC charger connected in parallel: the relay will put the alternator voltage (and with that also the starter battery voltage) over the terminals of the aux batteries, minus cable losses and such. The voltage over the aux batteries will be X at that point and give Y current flowing into the batteries. Assuming that the alternator CAN provide more power, the DC-DC charger is not bound by the output voltage of the alternator and the buck-boost circuit will up the voltage over the aux battery terminals to the point that it reaches it maximum output. Ergo, a DC-DC charger will always do it's maximum output even if you have a direct starter to aux battery of some sort. Makes perfect sense but perhaps someone will find it written out handy

Over the months I have been in contact with CTEK and some nice little nuggets of info stuck in my brain that might also be of use to others:

• the D250SE will prioritize solar over alternator
• they can be paralleled (makes sense with my above text)

This basically means that if you have solar it will use all up all of the available solar watts before it seeks for power from the alternator and if you have more solar on the roof than the unit can use (or require more DC-DC power) you can simply add another unit to your existing system. Yes you could also add one from another brand I reckon, but having two units with different charging algorithms would not be my preferred choice.

To conclude:

• limiting charge current with cables is certainly possible but a fairly extensive puzzle to solve
• I have not seen any evidence that my setup would be unsafe in any form or way; the cable sizes vs losses etc. are well within safety specs
• Once dialed in however there is no guarantee that it would work for a different battery setup (they don't last forever so would need changing at some point)
• Oversized alternators require scrutiny, they are not all created equally and
• The default wiring in the car is certainly not up to the task for such a device (ie alternator B+ and grounding cables).

AGAIN: IN MY SETUP! CHECK YOUR OWN SITUATION AND DON'T JUST COPY ME. You are free of course to use all my learned lessons and I would always be willing to help if you have questions

Whats left to do / ponder / consider? (last bullet list, I promise! )

• I am considering increasing my solar capacity which would automatically increase my DC-DC capacity.
• this would still not be enough but perhaps going full DC-DC and no smartpass/relay would be viable (remember weight is my biggest concern, cost my second)
• mixing and matching is not preffered. This means I would need to get a sterling power DC-DC and a victron MPPT for instance and ditch the CTEK stuff.
• I am not sure that the cable weight for the current limiting (extra length etc.) would weigh up to the above setup, in fact I think using equipment is heavier and certainly more expensive
• running electric AC is one of the most challenging electrical puzzles one can give oneself!
• since above; I have a free spot on the engine to run two alternators. Might be worth it so that I could run two factory standard units in stead of one expensive custom unit...
• The second spot does not however have the same airflow as the first.
• uneven loading would occur if you do not have two exacte same length cables.

Well, that's it for now. I'll give it a second go to find the topic regarding alternative alternators to update that as well.

Cheers,
-P