The Ultimate DCS 12V Dual Battery Setup for your 4WD! Detailed analysis of under bonnet applications.
We get asked all the time, is it okay to install the DCS lithium batteries under vehicle bonnets? In short yes absolutely. Why wouldn’t you when the DCS LFP cells have a wider operating temperature range compared with AGM cells are safer more robust and less likely to degas at extreme temperatures and have a significantly longer lifespan. Let us further elaborate below on the facts.
AGM cells have been traditionally used for high cold cranking amp requirements as they have good cold temperature characteristics compared to other lead acid cells. However on the flip side they don’t like the heat. AGM’s and other sealed type batteries (SLA or VRLA) do not like heat and should be installed away from engine compartments. Manufacturers actually recommend to stop charging when the batteries are subject to temperatures above 50 degrees C as the cells degrade significantly with temperatures already above 25 Degrees C.
Lithium Iron Phosphate (LiFePO4 or LFP) cells have a significantly higher cycle life than AGM cells especially under deep cycle discharge applications. The disparity is further increased as ambient temperatures increase. The cycle life of each chemistry can be increased by limiting the depth of discharge (DoD), discharge rate, and temperature, but lead acid is much more sensitive to each of these factors.
The below graph shows the capacity retention % Vs cycle life of AGM cells and Lithium LFP cells, when subject to moderate/normal temperatures (25 Degrees C). Which is also the temperature at which manufacturers specify their cell capacities.
As cycle life is influenced by the depth of discharge, the figure shows multiple DOD percentages for the lead acid. It can be seen that the AGM pack must be limited to a 30% depth of discharge to get comparable life to a lithium-ion that is at 75% depth of discharge. This means that the AGM battery must be 2.5 times larger in capacity than the lithium-ion to get comparable life at normal temperatures.
Generally speaking at DCS we usually tell customers to work on a 2:1 ratio. For every 200Ah of AGM battery you only need 100Ah of DCS LFP for similar usable storage. So what happens under extreme under bonnet temperatures you must be wondering? Thing certainly do heat up.
The below graph shows average temperatures of 50 Degrees C. The disparity between AGM and LFP is further exacerbated.
It can be seen that the AGM pack even when limited to a 30% depth of discharge (DOD) can only do around 1700 cycles with 60% capacity retention. The lithium-ion cell that is cycling all the way down to a 75% depth of discharge is capable of 2000 cycles with a 80% capacity retention. The AGM cell would need to be limited to around 5% DOD to have a chance. This means that the AGM battery must be at least 4 times larger in capacity than the lithium-ion to get comparable life at these extreme temperatures.
So why have so many 4WD enthusiast been installing AGM batteries under bonnets for years? And complaining about the terrible lifespan? Is it a lack of knowledge or misleading information being passed on by those national based battery retailers? One can only speculate, but at least now there is a superior deep cycle 12V battery on the market the DCS 12V lithium batteries.
Just to clear up some myths;
1. It is a myth that LFP (Lithium Iron Phosphate) batteries are more dangerous under extreme temperatures. In fact it’s the exact opposite. LFP cells have a much wider, safer and robust operating temperature range as can be seen in the above graphs.
2. Apparently LFP batteries cannot be connected directly to vehicle alternators as there is a possibility of overcharging the battery on a long drive. The DCS LFP batteries have an internal BMS (Battery Management System) even if the voltage regulator malfunctions in the alternator there is no possibility of overcharging the DCS batteries on long drives. Most modern vehicles are now fitted with “smart alternators” and these are even better for the DCS 12V batteries as they will maintain a lower overall voltage on the battery packs which will only further improve the lifespan. Remember from the graphs above "The cycle life of each chemistry can be increased by limiting the depth of discharge (DoD)”. A smart alternator does exactly that by limiting the charging voltages to conserve fuel.
So why are the DCS 12V 75Ah batteries so popular for under bonnet applications? It’s because under extreme bonnet temperatures a single DCS 12V 75Ah battery can deliver similar usable energy to that of a AGM 12V 300Ah battery. With significantly less weight and room. You don’t need huge 100Ah & 120Ah batteries under your bonnet.
Recently we upgraded a Mitsubishi Challenger 2014 dual battery system. The customer arrived with;
- 1 x Optima D34 cranking battery
- 1 x Ultimate 120Ah AGM deep cycle battery
- RedArc Isolator with externally mounted switch
- Total weight: 20KGs + 35KGs = 55KGs.
Chocablock packed in there. The customer was complaining that the cranking battery was struggling after only 2 years and he had the override switch on most of the time to crank off both batteries. With what you have just learned, that is no surprise is it? Also the amount of space the batteries had been taking up the engine cooling performance would have been significantly reduced due a lack of air flow and the under bonnet temperatures would have been at extreme levels.
We removed both batteries and replaced with 2 x DCS 12V 75Ah batteries. One for engine cranking and one for auxiliary deep cycling. Is that enough power? Absolutely. With 750CCA these little batteries will easily crank over even the largest of V8 turbo diesels.
- 2 x DCS 12V 75Ah batteries
- Matson 140Amp Dual Battery isolator with integrated switch
With the isolation cut off voltage being 12.8V means that the cranking battery will be able to deep cycle down to around 80% of it’s capacity before it’s isolated. At 12.8V the battery will crank over the engine. So with the auxiliary 75Ah + cranking 60Ah (80% DOD) = 135Ah of combined usable storage. For deep cycle accessories as a dual battery setup this actually more than doubled the existing AGM battery setup which only had a storage capacity of 120Ah with 50% being usable = 60Ah.
135Ah usable storage with the lithium's Vs only 60Ah with the AGM's.
Overall weight dropped from 55KGs to only 20KGs. That’s a weight saving of 35KGs off the nose of the vehicle which will keep the suspension happier.
The DCS batteries are connected directly to the alternator which allows the batteries to swallow everything the factory 80Amp alternator can provide, which would be a maximum of around 60-70Amps. These batteries can recharge super fast and are designed to be able to recharge at a continuous 100Amps.
With the superior lifespan of the DCS LFP cells we don’t expect to see this customer again for at least the next 10 years. Or maybe his grandchildren if this beast of a tourer gets handed down.