My OEM battery tender died, looks like it got wet somehow. Going to get another one, but instead of the CTEK, was thinking of something a bit more universal. I can get a 16/8/4A 6 stage charger https://www.projecta.com.au/battery-...attery-charger. Is there a maximum charge current I should be worried about?

For my Unimog Camper truck I am in the process of changing all the batteries in the truck over to Lithium. The house batteries to Lithium LiFePO4 4 x 120AH. https://itechworld.com.au/collection...-max-discharge. They will replace 6 x 75A Optima batteries, and give me extra capacity and save over 100kg of weight as well.

The cranking battery I am going with a Lithium Ion https://itechworld.com.au/collection...h1420-cranking. It weighs 8kg and will replace a 38kg battery.

I was thinking that when it was time to change over the batteries on the Bentley, maybe I will change the Lead acid over to Lithium as well using the batteries. The LiFePO4 for the bigger house battery, and the Lithium Ion for the starting battery. Just wanted to know if anyone has done something like that.

 

I thought they are both AGM anyhow ?
I would replace when the time comes wit exactly the same OEM spec .Theses cars are uber sensitive to battery weakness.
Any small ah trickle charger will do , the drain when parked is pretty big .

 

I have AGMs in my truck, they work OK, but a 120AH Lithium is the same as a 200AH AGM, LiFePO4 have a lot more capacity and are better at holding a higher voltage for a lot longer. The Lithium Ion cranking battery will provide 1500CCA and not drop below 12V. I was thinking that providing twice as much reserve current capacity for the Main battery and over twice the cranking power than the OEM AGM battery may be a good thing.

On my Unimog, even fully charged the AGM 900CCA battery drops down to 11V when cranking, which makes it draw a lot more current. If the battery is not fully charged, it drops below 10V and has a huge amount of current, which can burns out the solenoid contacts on the starter motor.

 

Bentley GT / FS forums are full of battery woes from folks who thought they new better .If they weren’t then who cares as anything goes ?
Theses cars are incredibly battery sensitive.

 

Don't mess with the batteries. You'll be sorry and haunted by weird car faults.

 

That was the idea, replace the AGMs with something a lot more reliable and more stable.

 

I see your point Iain.

Has anyone actually done what Iain is suggesting ?

 

My biggest concern is that the alternator's behaviour will have been optimised, in terms of voltage and current, for the type of battery for which the car is designed. For example with JLR products made after the late noughties you must reconfigure the car (using JLP's SDD diagnostic tool) if you change battery, and especially if you swap from AGM to regular lead-acid.

My understanding is that Lithium batteries require an entirely different charging regime from lead acid, and unless you can mitigate that difference, possibly by using a different regulator on the alternator then you may be in for trouble.

I fully understand and agree with your sentiment about improved supply voltage stability during cranking being a good thing for all other consumers on the battery bus (i.e. control modules various dotted around the car), but I'm sorry but you're going to have to explain how a reduced voltage can result in more current flowing.

When static the winding resistance of a motor is pretty much constant, hence according to Ohm's law, the current which can flow it will equal V/R. Thus the greater the voltage, the higher the current. Conversely the lower the voltage, the lower the current.

As soon as that current excites the motor, then the motor starts to turn, consequently a back emf builds up in series with the winding resistance, thereby reducing the current that can flow.

If you're getting contact burning because of a weak battery, I would suggest that it is happening because the contactor isn't pulling in so quickly and efficiently leading to arcing across the contacts during the transition....

 

The Unimog is a very basic thing electrically, I actually only need three wires to make it run, and there is a way to start it without any power at all. Not sure why the current draw gets higher, can only say what I measured with a simple clamp on amp meter. Maybe the high compression on the diesel needs a certain amount of power to crank, I have a reduction gear starter on it, but if it needs a minimum power output to work, given P=VI, if voltage is low, then to get the same power, current has to increase to be able to turn over the motor. I'm a Civil Engineer, I'll leave the calcs to the Electrical engineers

The Lithium batteries I have bought are set up for use with "normal" alternators, like the low tech versions I have on the truck, that's why I bought them.

My Discovery 3 has a "smart alternator" which can put out 15V depending on what the battery needs I understand it, the regulator is capable of varying current and voltage controller by some fancy electronics in order to save fuel.

Is the Hitachi alternator on the Bentley a "smart alternator"> Is it capable of varying the current to save fuel, not something I think Bentley would worry about.

 

Unfortunately it doesn't work like that: the supply voltage and current drawn are a function of the battery, the cables, the contactor and the load (motor).
Once you have measured the potential difference at the motor and the current flowing through it, only then can you determine how much power it is consuming.
You can't say "it's a 4800 Watt motor so if I supply it with 12V it will draw 400 Amperes, but if I supply it from a 6V battery then it will draw 800 Amperes".
Otherwise if you extrapolated that theory then with a OV (flat) battery it could draw an infinite current.

Ordinarily, when operating within its design limits, a motor produces an amount of torque (in Nm) which, when multiplied by 2 x Pi x Speed (rps), gives you the power (kW) being generated.
So long as the torque produced by the motor is significantly greater than the combined compression and friction torque of the engine resisting it, then the engine will turn and then start.

However, if the torque needed to turn the engine gets close to, and/or exceeds the motor's ability to deliver torque, then as the motor approaches its stall point, obviously its slows down.

As the motor slows down, so the back emf which I mentioned earlier decreases, thus the current starts to rise, and heat is generated inside the motor windings.

What you are seeing with your ammeter is that increase in current due to decreased speed, and that is not the region where you want to be operating the motor.

Sorry, that I don't know without looking at the circuit diagrams.