Originally Posted by CJZA

Thanks for the input! I will stick with stock size for now and see how it goes. There are so many variables to consider that it is next to impossible to do a calculation and come up with a simple answer. Worst case I will kill the AGM start battery sooner rather than later.

Just FYI - AGM and Lithium have very different charge absorption and discharge rates. Lithium has a very low internal resistance and is generally rated for 1C discharge - meaning you can discharge a 100Ah battery at 100A for one hour without damaging the battery. It will also take a 100A charge rate meaning a 100AH battery could be recharged in as little as one hour (this is not recommended for longevity). AGM on the other hand goes through stages - bulk, absorption and float. During the bulk phase, they can consume between 0.2C and 0.4C In your case, an 85Ah battery will draw between 17A(0.2C) and 34A(0.4C) after which they switch to the absorption phase and the charge current starts to drop off quite steeply. The closer they get to full charge, the less current is consumed - in the end only around 1A or 2A. This is actually great for the alternator and the starter battery as there is time to meet the charging needs of the secondary battery. You can read more about it here - https://marinehowto.com/how-fast-can...ry-be-charged/

Obviously the downside to AGM (from our perspective) as a secondary battery, is the time it takes to recharge the battery due to the charging characteristics, the weight of the battery, and the fact that you shouldn't discharge an AGM to less than 50% if you want to ensure a long life. This means if you have a fridge that draws 2.5A (eco setting) and runs for 30min per hour (consumption of 1.25Ah), you would be able to run said fridge for 40h on a 100Ah AGM battery, whilst you could run the same fridge for 80h on a 100Ah Lithium battery. Discharging lithium to 100% isn't recommended either, so lets say we will only discharge it to 20% SOC, giving us 64h runtime out of our battery. To equal that in an AGM Battery, we would need a 160Ah AGM Battery.

Recharging the batteries once we get going - via the vehicle - we could recharge the Lithium from 20% SOC to 100% SOC using a 50A DC to DC Charger in as little as 1h40m, whilst recharging the 100Ah AGM from 50% SOC to 100% SOC could take 5h assuming a 0.4C charge rate and constant Voltage supply. The same holds true for solar in terms of time needed to recharge a battery, so ultimately there are a lot of benefits to Lithium, and that's why I chose it as a secondary battery.

BUT because Lithium has such low resistance, it is a current hog, and will happily suck all the current it can until it is fully recharged. This isn't great on shorter drives, as you have a huge load (50A) on top of charging the starter battery which will enter its Bulk charge stage requiring between 20A and 35A as soon as the vehicle starts. Add to this the requirements of heated seats, heated windscreen and AC (All used from the moment the vehicle starts), and it obvious that doing this (short drives only) repeatedly over a couple of weeks/months and not putting the starter battery on a tender will most likely kill the AGM starter battery pretty soon.

Batteries are there to act as a shock absorber for the electrical system and also as a means to run the starter motor before the alternator gets going. An electrical system should always be designed for worse case scenario in terms of current draw and headroom added on top of that for safety. That means the Alternator should be able to cope with the entire load of the system in terms of its output at IDLE. Inthe case of a winch, the starter battery will supply the winch needs only up to a point. Winches are irregular draws on the system, so unless you are winching multiple times per day, whilst barely idling, you should be ok.

In my case, there are things that I could do to mitigate the draw:

• Set the DC to DC to deliver a lower output to the Lithium battery. Say 30A instead of 50A (in the ideal world, there would be a smart DC to DC charger that is connected to a shunt on the starter battery, and adjusts its draw/output to account for vehicle draw - in effect sensing when it can ramp up, and ramping down when the vehicle needs more power from the alternator),
• Add solar panels to keep the Lithium battery topped off (there are drawbacks to the battery life doing this), reducing the need to run the DC to DC charger,
• Have the DC to DC charger switched in the cab, so it can be disconnected in cases where the winch needs to be used for example.

All this being said, I may still end up with a larger alternator because I choose to only go wheeling in the hottest months, run my entire fridge as a freezer so I can eat Ice cream multiple times per day, get myself in over my head on the wrong trails and have to winch twice as much, camp under trees for days at a time, refuse to get a second/larger secondary battery etc. Running a larger alternator on a system with larger demands is good practice. No alternator will provide its maximum output rating constantly. As it works, it heats up, and output starts dropping off. A 50% duty cycle on a 250A alternator is better than an 83% duty cycle on a 150A alternator.

This is longwinded, I know. The point is, just because something works one way for one person, doesn't mean it will work for someone else. Your system may be very different to mine, so you cant say "I don't need a high output alternator so you don't either". There are so many differences to account for - You run a FLA as starter, AGM as secondary battery (most likely wired up to the starter battery seeing that you have a traxide installed) which means you have a much larger shock absorber/reserve to draw from when you winch. You are not putting a 85A load on your battery/alternator just because your traxide is rated to 85A. Because of your battery chemistry and the way it is wired up, you wont have a constant 50A draw on your system or alternator either, your largest load will be during bulk charging and it will be short. My lithium battery is also not wired into the system as a supply. It is only there to run my fridge/camping accessories, and does not act as an additional shock absorber/reserve during high draw situations like idling whilst running the AC whilst winching.

All of the factors have to be taken into account. I may very well be overthinking this, but it has been a very enlightening exercise and the questions have made me read more and dive deeper into the topic. Thank you!