Dual Battery System - AGM vs Lithium Thoughts

**Strop** · 15th November 2023, 09:40 PM

Hi all.

This is long and I apologise in advance. It has also been dealt with in other posts but I do not believe in this way.

I am about to install a dual battery system into the, new to me, D5 but I am now re-thinking about which way to go - AGM or Lithium. As I am currently thinking I can see that there are the two sides in this debate - AGM with a Traxide System (had one in my D4 and was happy) as opposed to the lithium with a DC-DC unit. I have done a fair bit of reading on this so I thought I would put my thinking down on paper and have you experienced people critique my thought process.

To me there are four issues in the decision making process:

Weight
Car Power Usage
Charging Time
Price

Weight.

The Lithium batteries are a clear winner here. With a Lithium you get about 9 amp/hrs per kg as opposed to AGM 3.75 amp/hrs per kg. Gets even worse for AGM if I consider that I can discharge a lithium battery to 20% of its power whilst some say 30% for AGM others 50%. I am working on 30% as the limit. The mounting for them is the same weight so it is the weight difference between the batteries that is the saving. The main use for my battery is a fridge. It uses about 3 amps per hour so about 72 amp/hours per day. So I need about 120 A/H AGM (32 Kg) or 100 A/H Lithium (12 Kg).

The weight difference is about 20 Kg.

Tim (Traxide) would say that I don’t need the full 120 A/H battery because the traxide unit would use some of the main battery. Lets say the 95 Amp/Hr battery could not go below 50% charged so there is 40 Amp/Hrs of battery for use. That means I only need to have an 80 Amp/Hr auxiliary battery. The SSB HVT-70LD is 85 amp/hrs and weighs 23 Kg so our weight difference is back to 11 Kg. The mounting hardware I was considering using would add about another 3 Kg if I went Lithium and the Lithium DC-DC charger about another 1 Kg. So let’s say the weight difference is down to 7 Kg. Not as much as we first started with but perhaps enough to sway a decision.

Car Power Usage

The car uses power from the battery. I watched a youtube video and did some reading on battery usage on the new Defender. I would love to be corrected but I think the Discovery has a very similar electrical system. What set me thinking this was the warning message I was getting about low battery charge on my, new to me, car. I open the doors a few times and I am getting this message. The video outlined how opening any door set the system alive and it began to do its thing. Long story short is that the 2 videos are worth a look but it concludes that about over 2 amp/hrs are used every time you open a door (system set in motion for 13 minutes). Out camping, open the door 5 times - let’s say you lose 15 amp/hrs. If, as I suspect my battery is not 100% then I do not have enough battery power as described above.

So the 85 amp/hr battery is no longer big enough. Impacts on my weights.

To be on the safe side there is the SSB HVT-70ZZD at 105 Amp/Hrs. Weighs 27 Kg so we are back up to 11 Kg difference.

Interestingly if you leave the back open (as I do) but the open a door the system resets and begins to use power. I know, leave the door open and only close it once a day. That is not going to happen and it is not just kids. I am amazed at how many time I go to the car to get something and habit has me closing the doors all the time.

Does this give an advantage to the lithium system as it is not connected to the main battery? Perhaps, but I have read people advising others to carry a jump start kit for those times the car eats itself alive. Some consider it an essential piece of kit to carry.

The Traxide system allows the auxiliary battery to help the main battery. A great idea as long as both batteries are not dead and a jump start required.

Charging Time.

This one I am having some difficulty getting my head around, especially about AGM batteries.

Let’s start with Lithium. A DC-DC charger states what is its maximum charge rate. 50 amps sounds ok but from my reading it is not as simple as that. There is the possibility that with a smart alternator once the primary battery reaches full charge that the alternator will reduce how much power it is delivering hence lowering the current going to the DC-DC charger. Unless you wire the DC-DC charger up with an ignition switch. Not so sure about this part of the process. So a bit hard to determine how long it will take to charge up a battery.

Next is the AGM with a Traxide system. I had always found with my D4 that I could go for a beach drive or out onto a track for a while and all was fully charged by the time I get back. The problem I am having with understanding this side of it is that I look at AGM batteries and they state things such as 23/30/35 amp charging rate. Tim (Traxide) talks about a much higher charge rate from the alternator directly to the batteries through his units. A lithium battery clearly states it can take a charge rate of 100 amps but it is not so clear cut with AGM when I read specs. I need help understanding this part. Can the alternator produced 200 amps and shove 100 into the main battery and 100 into the auxiliary (numbers used for example purposes only).

As far as solar goes both units will get charged at the same rate as the panels you are using. Lithium will use the inbuilt MMPT controller and the Traxide unit will use the solar controller that came with your solar panels. If I work on a panel that produces 10 amps I need to have it out in the sun for 8 hrs to replace what the fridge uses. simple maths here.

Cost:

AGM wins on initial purchase price here hands down. The argument is about how many more cycles you get out of your lithium system. I don’t intend to have the car in 10 years time and I will be too old by then to do some of the tings I do now so I can replace the AGM batteries perhaps once and still be in front. Really depends on the brand of lithium battery you buy. Kings 100 Amp/Hr is on special as I type for $400. 120 A/H AGM from Kings $199. I just use their prices as an example. Other lithiums are $1000.

DC-DC charger and Traxide system similar pricing so not a big issue to consider.

The Questions.

So is there something that I have missed? Is the electrical system of the D5 a power eater the same as the Defender.

Whilst weight was a major consideration for me with the D4 it is less of one with the D5 but, still a consideration. 11Kg is not make or break at this time but 20 may be.

I think the key is how you use your battery system and the charging requirements you therefore require. As I said, I mainly use it for just a fridge and incidental charging of things like camera and phone batteries. Though I tend to make sure I do these things when driving along. I can be at camp for days at a time but my solar blanket in the past has kept everything charged up. If you are driving on the road ALL the time either system will probably be OK as it only has to last you the night. For longer stays on those days when it has been raining a drive to charge things up has helped but I can not from my personal experience comment on how long the drive would take with a DC-DC charger.

So, please let me know if anything I have written is incorrect or needs further clarification. I am still in the process of making my mind up.

**drivesafe** · 16th November 2023, 06:20 AM

Hi and I’ll just make this short for now.

Your D5 system is nothing like your new Defender but is very similar to your D4.

The batteries I recommend are cranking batteries, not deep cycle batteries, and the reason for this is the high current fast charging you can safely do with a cranking battery.

The SSB HVT range of batteries have a 100% usage capability with a good cycle rate, and while I usually only use about 85% but you can set your Traxide isolator to give 100% usage if you wish.

Because of the way the Traxide isolators work. They usually extend the operating liflespan of your cranking battery, so a saving long term there.

Again because of the way the Traxide system works, your cranking battery is likely to be in a far better condition at any time and as such, is far less likely to need a jump start, but should you ne to, with your D5 and a Traxide setup, just one jumper lead, connecting the positive ( + ) of both batteries in the rear cargo area, and you are done.

I’ll post more later, but one unrelated comment, I will NOT be making a dual battery setup for the new Defenders at this time, because of the complexity of their electrical charging system.

**Strop** · 18th November 2023, 09:56 AM

Thanks Tim.

As a continuation I thought I would ask about batteries. I actually find Lithium batteries easier to understand but that is probably because of my lack of knowledge and their bigger numbers.

Having said that let me start with AGM batteries. Since your comment Tim I have looked at a lot of batteries. Many batteries state that there is a maximum charging amperage. I am assuming that this should not be exceeded by the charging device be it alternator or DC-DC charger. I also assume that is why you say use cranking batteries because they do not state a maximum charging amperage. So your recommendations are for the dual purpose batteries such as the ones from SSB HVT range or the Optima Gold Top range.

In looking at these batteries though they say you can discharge to 100% it also indicates that there is an impact on the number of charge cycles that you can expect. Quite a big difference when you look at 30% discharge compared to 100% discharge but you could probably get 3 years out of them if you hammer them. If you only use them every now and then, say 2 weekends a month then they should last significantly longer. I kind of thought that it would end up less than $100 per year in costs of the battery. With a warranty of 2 or 3 years I see them as good value. My lithium batteries in my van are 3 yrs old and they are my only experience with that type of battery. It will be interesting to see their longevity.

In the AGM range of deep cycle batteries I have been looking at a number of slimline batteries simply because of their profile and their ability to fit into narrower spaces. In this area I have seen examples from Kings and Kickass. In both cases these batteries state a maximum charging rate of around 25 amps. This would mean that if using the alternator they are likely to get significantly higher charge rates than the manufacturers recommendations. Similarly a 40 or 50 amp DC-DC charger would be too big.

I am assuming that exceeding the maximum charge rates would produce more gas that could potentially be vented or other side effects. Not sure this is a good thing for a battery that is mounted inside the car. Perhaps not as big an issue if mounted under the bonnet or in a rear tray. Perhaps someone could let me know what happens to these batteries that receive too much charge and is it dangerous to have inside the vehicle. Does it shorten the lifespan of the battery?

With the lithium I can see that even the most basic are capable of taking a larger maximum charging rate though again it would be essential to check that the battery can take the current provided by the DC-DC charger. In many cases 25 amps might be the limit. Simarly what happens if you overcharge a lithium battery? Fire, expolosions… any thing else? I was thinking that the internal battery BMS might simply switch them off and not allow them to charge.

So in thinking about DC-DC chargers in reality to AGM batteries, you can’t use a 50 amp unit you are really limited to a 25 amp one. This means charging a battery up will take even longer. Probably similar to some lithium batteries.

I was surprised to see that the Redarc BCDC chargers only bulk charge for 2 hrs before switching to float. Ok if you have a 100 Amp/Hr Lithium battery using a 50 amp/hr charger but not ok if you are using a 25 amp/hr one. So anything bigger than 100 amp/hrs would probably require multiple BCDC chargers. I have not seen the 2 hr limit with other brands of DC-DC chargers. Having multiple 50 amp DC-DC chargers to allow quicker charging whilst driving would significantly increase costs.

If you are using solar charging I don’t think type of battery has any impact on decision making here. What you need to know for any solar installation is how much energy do you need to replace in a 6 hr period (winter). If I am stationary and I need 100 amp/hrs a day to keep things going I need solar panels that produce 16 amps - say a 200w panel. I would suggest those figures are not going to impact on the specs of maximum charging amps for any type of battery.

One thing I have not looked at but should is the ability of the battery system to run a small inverter. A 500 watt inverter would be pulling about 42 amps. Most batteries I think should be able to do this. If you go to a 1000 watt inverter to run a small microwave it may be different. I don’t use an inverter in the car but just looking at possibilities. The quick look I had at AGM batteries they state a max discharge rate not a continuous one.

So, looking at batteries and charging systems am I correct in saying that it really comes down to how you use your vehicle. If you are driving every day (crossing Simpson Desert) or alternatively camping somewhere for more than a few days it will impact on what kind of system you choose. Similarly, if you live in Nth Queensland as opposed to Tasmania the selection might change.

I have asked a couple of questions regarding battery chemistry here and I would appreciate any comments on what happens.

**drivesafe** · 19th November 2023, 05:05 PM

Hi again Laurie and I am a little tired up at the moment but will try to explain a little about lead acid batteries so you might be able pick a good battery from one you probably don’t want.

First some battery history to help explain good from bad.

The first AGM batteries came into RV use a little over 20 years ago but they were not intended for RV market.

These batteries were designed as replacements for deep cycle batteries used in the RAPS ( Remote Area Power Supply ) industry.

The batteries they were replacing were glass batteries, so the original deep cycle batteries were a No-No for RV use.

Once these new AGM deep cycle batteries became common use, people got the idea to use them in RV situations.

But they were never designed for the purpose and had a number of limitations, such as being fragile, not tolerating high temperatures, low current charging capability and a maximum charge voltage of 14.4v

There had a maximum charge current of just 20% of the battery’s total capacity. So a 100Ah AGM had a maximum charge current of just 20 amp.

With these limitation, people would try to mount them under the bonnet of their 4x4 and wondered why they never lasted 6 months.

The manufacturers of these batteries quickly realised there was a huge potential market for the right type of AGM and started improving there specs.

First improvement made was to raise the charge current capacity, and this slowly rose from the initial 20% to todays 35 to 40% for good quality AGM Deep Cycle batteries.

So that 25% shown for those batteries you have posted up, is not that good and hence the cheaper price.

More to come but I need a break.

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