Lithium 12v auxiliary system in a vehicle

Electronics
#1

I’m currently getting ready to redo the electrical in my truck. I took out the few hundred Ah worth of AGM batteries a few months ago, and now want to get the lithium replacements going.

I am looking for the best way to handle the charging / discharging of the system. I would like to be able to have a charger that operates off the alternator when the vehicle is running, and off 120/208 when parked.

I’m not as familiar with lithium systems, but I figure I could collect the knowledge from here. I have asked around before and gotten a few leads but was terrible at taking notes :confused:

Most of the onboard chargers I’ve found operate in the 24v low end, which would mean adding some very beefy buck converters to the output for the 12v vehicle system, although it would allow for a 24v inverter rather than my current 12 (more efficient).

I currently have 8x large batteries (was planning to run 4s2p to replace the 12v AGMs, since most equipment will operate up to 14.2v), but can also run 8s if I go with 24v equipment)

I’ve built SLA/AGM charging equipment before and typical regulator systems, but I’m not familiar with lithium charge curves and making buck converters that beefy.

Cheers,
-Jim

#2

I’m sure you know this, but it’s worth stating for posterity; lithium ion batteries are a fire hazard, especially when used in large banks as their charging and discharging becomes much more complex.

Typical single cell chargers run constant current to about 4V, then constant voltage to 4.2V. If keeping charge cycles maximized is a priority, only charge them to 4V or 4.1V. Running multiple cells in series just multiplies the voltage.

The largest challenge is going to be cell balancing and thermal management/checking. It is imperative to ensure that cells neither over charge (fire) or over discharge (anode dissolution who’s deposits can form dendrites which can cause a fire). Typical small integrated systems will have a IC that monitors voltage and current, and sometimes heat, of each cell with a FET to turn off the pack to prevent failure. If cells get out of whack in regards to capacity, this can also cause a fire. For example, if when two cells in series are discharged and they should be at 10% capacity and in reality one is at 0% and the other is at 20%, when charged one will be full and the other will be overcharged.

Please make sure that your batteries are rated for the current draw for your application as well. They typically will have a C rating for both charge and discharge. 1C = 1x capacity. So a 1C discharge of a 2AH cell = 2A max, 2C would then be 4A, 0.5C Would be 1A. Exceeding the current draw or charge can lead to a fire.

All in all, if a lithium ion battery gets too hot, discharges too fast (internal or external short), gets physically damaged (internal short), or gets overcharged, they are likely to go into thermal runaway and the results can be catastrophic.

In case you haven’t seen on, a ~2A lithium ion battery (18650), when going through thermal runaway, goes off like a small model rocket engine. They get hot enough to melt aluminum, but I’ve not seen melted copper or steel from a single cell going up. If you’ve not seen one, I suggest looking up videos on youtube.

I’m not trying to dissuade you from using lithium ion cells, just trying to inform people of the dangers. Some things are inherently dangerous, like welding or driving. That doesn’t mean we shouldn’t do them, just that we probably shouldn’t do them in the nude.

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#3

What type of Lithium batteries are you using @hon1nbo ? There’s a few different chemistries and some are much safer than others.

#4

There are LiFeP04 batteries in common lead-acid formfactors inbuilt BMS’s that are compatible with lead-acid chargers/electronics, however they tend to be eye-wateringly expensive.

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#5

The cells @hon1nbo got from me are LiFePO4, which are much, MUCH safer than LiPO.

LiFePO4 cells are happiest when never discharged below 20% or charged above 90%. They must NEVER be charged when they are below freezing. They self heat when discharging and charging, so if you start charging immediately after running in cold weather you’ll be fine.

Standard Lithium chargers do a constant current until a set voltage is reached, then start dropping the current to stay at the set voltage until a set minimum current is reached. I’ve had very good luck by charging at a set current, then stopping the charge at the set voltage. This leaves 10-15% of the top charge off, but greatly simplifies charging. You simply control a contactor to disconnect the battery from the charger and power down the charger when the set voltage is reached. The specific chemistry and manufacturing process control both the charging current and voltage and the max discharge current and voltage.

I’ve used one of these to control contactors for a charging safety circuit.
https://www.lightobject.com/Programmable-Digital-AH-Meter-Blue-LED-Ideal-for-Battery-Monitoring-P278.aspx

ETA: I’ve seen on the internet where some people have used this method to control old transformer based high power lead acid chargers to charge LiFePO4 packs until they upgrade to a switched power supply charger.

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#6

I was interested on why not to charge below freezing… I found this …

When charging at above-freezing temperatures, the lithium ions inside the battery are soaked up as in a sponge by the porous graphite that makes up the anode, the negative terminal of the battery. Below freezing, however, the lithium ions aren’t efficiently captured by the anode. Instead, many lithium ions coat the surface of the anode, a process called lithium plating , which means there’s less lithium available to cause the flow of electricity and the battery’s capacity drops. Charging below 0°C at an inappropriate charge rate, also causes the battery to become less mechanically stable and more prone to sudden failure.

The damage to the battery when charging at colder temperatures is proportional to the charging rate. Charging at a much slower rate can reduce the damage, but this is rarely a practical solution. In most cases, if a lithium-ion battery is charged below freezing even once, it will be permanently damaged and must be safely discarded or recycled.

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