Thread: Discovery 2 Dual Battery

Originally Posted by d2dave

A Lithium on the other hand, regardless of state of charge, will take what ever you throw at it.

Not in EVs, the higher the state of the charge, the slower the charge rate, although that could be the EV BMS?

Originally Posted by Bohica

Not in EVs, the higher the state of the charge, the slower the charge rate,

Can't have the things blowing up, now can we?

A lot of care must be used when selecting a lithium battery as an auxiliary battery.

SPECIAL NOTE, lithium batteries should NEVER be used as a cranking battery.

The primary difference between charging a lead acid battery and a lithium battery is that a lead acid battery requires three stages of charging, Bulk, Absorption and a Float charge.

Whereas Lithium Batteries require two stages of charge, Bulk and Absorption, and once charged, the charging source must turned off.

ALSO NOTE it is very rare for a Lithium battery’s BMS to act as a current limiter during charging, In other words, the lithium battery’s BMS does not shut down the battery once it is fully charged.

Many Lithium Battery BMS will shut the battery down if there is a high current DISCHARGE, this has nothing to do with charging.

The above info relates to charging with a DC/DC device, Solar Regulator and a 240vac Battery Charger, it has little to do with charging from an alternator.


ALTERNATOR CHARGING OF LITHIUM BATTERIES.
There are a number of specific requirements when charging lithium batteries directly from an alternator.

First and foremost, when selecting a lithium battery for charging directly from an alternator, with a Discovery 2 or 3, the battery MUST HAVE a CONTINUOUS CHARGE CURRENT rate of at least 100 amps.

NOTE this is NOT the MAXIMUM charge current makes on a lithium battery, as this is something a lithium battery can only tolerate for a very short time of a few seconds to a few minutes.

If you have discharged a lithium battery to a low state and you then drive for a few hours, the battery could be receiving a charge current as high as 100 amps for quite some time.

Note, this does not apply to a Discovery 4 or any vehicle with a SMART alternator function.

I have tested charging a Lithium battery with a 140 amp alternator and with a 100Ah lithium battery mounted in the rear of the vehicle and Twin 6B&S ( 13.5x2 ) cabling connecting the lithium battery, via a DT90 isolator, to the cranking battery.

While I monitored the currents in both directions ( Charging and discharging ) I would periodically blow a 50 amp fuse but an 80 amp fuse allowed 70+ amps of continuous charging and up to 140 amp discharging while starting the motor.

How fuses work is for another time.

I could change the operation of the alternator in the vehicle I used, from a SMART alternator function, similar to a D4, to a Variable Voltage alternator function, similar to how a D3 alternator works.

My tests were two fold. They were to see how my SC90 and DT90 isolators worked with a lithium axiality battery and how well a lithium battery could be charged with an alternator.

When charging a lithium battery from an alternator, you do not have to worry about shutting the charging off once the lithium battery is fully charged as this will happen every time you turn your motor off.

Constant charging of a fully charged lithium battery from an alternator would only be a problem if you were to regularly drive for 10 to 15 hours straight. Not likely to happen.

My findings were that my isolators worked fine with a lead acid cranking battery and a lithium auxiliary battery but I got some interesting results relating to using a Lithium Auxiliary battery.

First off, because lithium batteries have a higher and constant rested voltage ( around 13.2v -13.4v ) my isolators would remain on, regardless of whether they were set to Ignition mode or Shared mode.

So when I turned the motor off, with the lithium auxiliary battery sitting at 13.3v, and the lead acid cranking battery sitting anywhere from about 12.3v to 12.7v, the lithium would slowly discharge back into the cranking battery.


I used this setup for a little over two years.

The alternator operating in Variable Voltage mode, would really get much over 14.0v and regularly settled down to 13.5v, while driving.

This still allowed the lithium battery to be fully recharged ( 95% or higher ) even after short drives.

NOTE, the cranking battery was on its last legs and beginning to fail to start the motor, through age, when I first put the lithium battery in.

As I stated, the lithium battery was used in this setup for a little over two years, but with in a few days of installing the lithium auxiliary battery, the cranking battery’s starting performance improved dramatically.

After having the lithium auxiliary battery in for two years, I decided to remove it to carry out a fully monitored bench discharge test.

The lithium battery still had a 100% capacity.

But the surprising thing was that the “STUFFED” cranking battery lasted another 6 months and I only replaced it when my BM2 Bluetooth Battery Monitor warned me that the cranking battery voltage went too low when starting the motor.


Now when camping, with two lead acid batteries, the cranking battery and the auxiliary battery, while camping, your fridge, camp lighting, phone and computer charging, etc, current would be shared and come from both batteries.

With a lead acid cranking battery and a lithium auxiliary battery, all the power would initially be coming from the lithium battery even though the two batteries are still connected.

Only if the lithium battery was discharged down to about 15 to10% SoC, would power start coming from the cranking battery.

In this situation, the DT90 isolator still protected the cranking battery from being over discharged and the DT90 would completely shut down before the Lithium Battery’s BMS shut the Lithium battery down.

When you went for a drive, even with both batteries discharged to their lowest allowed level, both batteries would be back over 95% in about 90 minutes.

If you decide to use a lithium battery as an auxiliary battery, you could use two lithium batteries with a continuous charge current of 50 amps, as long as they are wired in PARALLEL.

I have also posted this info up in the VERANDAH section, in the On-Line Auto Electrical thread.

Originally Posted by Bohica

I've had a Traxide SC80 for about 6 years. A wonderful bit of kit. Why people use a DC - DC chargers is beyond me.

Depends where the second battery is, when it's at the opposite end of the car and volt drop starts to affect the maximum it'll charge to it they make more sense.

Originally Posted by admiralranga

Depends where the second battery is, when it's at the opposite end of the car and volt drop starts to affect the maximum it'll charge to it they make more sense.

My aux LiPO4 battery is in the rear luggage bin, driver's side. I have hefty cables. I get 14.6V at the aux battery, when the engine is running. I guess I do not need a DC-DC charger.

Originally Posted by admiralranga

Depends where the second battery is, when it's at the opposite end of the car and volt drop starts to affect the maximum it'll charge to it they make more sense.

As I posted, I was getting as much as 70+ amps at the Lithium auxiliary battery, and this was mounted in the boot. So it actually depends on cable size and as posted, I use 6B&S ( 13.5mm2 x 2 ).

I have set up caravans and again seen 73 amps at the house batteries in the van while charging with an alternator.

There are very few DC/DC devices that can deliver that sort of charging and they would not work at that current rate in a D2 because the DC/DC devices are so inefficient.

Originally Posted by Tins

Can't have the things blowing up, now can we?

True, I mean with 321 cars a day in the USA catching fire, we want to reduce the number that burn. Oh that is the number for petrol and diesel vehicles.
Data from Sweden had ICE vehicles have a 1 in 38,000 chance of catching fire, those numbers include hybrid cars. Ice vehicles have a 1 in 1300 chance of catching fire. Access Denied

Originally Posted by drivesafe

As I posted, I was getting as much as 70+ amps at the Lithium auxiliary battery, and this was mounted in the boot. So it actually depends on cable size and as posted, I use 6B&S ( 13.5mm2 x 2 ).

I have set up caravans and again seen 73 amps at the house batteries in the van while charging with an alternator.

There are very few DC/DC devices that can deliver that sort of charging and they would not work at that current rate in a D2 because the DC/DC devices are so inefficient.

I'm curious to know why a lot of people here are not so fond of DC-DC's? for years I've seen them to be the ideal choice for aux systems. Is it a price thing? e.g. isolators are much more cost effective?