Battle Born Cold Charging Study: Analysis 

Wednesday 14th October 2020 
by Matt 
 
Battle Born Cold Charging Study: Analysis

Can you use lithium batteries in cold temperatures? That's what Battle Born sought to find out in their study, and here's our analysis!

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Introduction

Oftentimes when we tell people about our Battle Born lithium batteries, the question will come up: can you use lithium batteries work in cold temperatures?

The simple answer is: yes.

However, there are a few nuances to the answer that have a tendency to trip people up. To try and provide a more comprehensive answer to this question, Battle Born Batteries has conducted a study comparing the performance of their 100Ah lithium batteries to a Group 31 AGM lead acid battery in a range of temperatures.

In this blog post, we'll take a look at the study and some of our own data to see if you should be concerned about cold temperatures when considering a battery upgrade for your RV.

Disclaimer

We are partners with Battle Born Batteries and articles on our site (including this one) contain affiliate links to their website.

We installed three Battle Born lithium batteries in our RV back in August 2018. We purchased these batteries (plus our inverter, etc) at full price from them after conducting our own research into the best solution for us.

RV Electrical Upgrade: Part 2 – Battle Born Lithium Batteries & Victron Inverter

In Part 2 of our series on upgrading the electrical system in our RV, we're going to be jumping straight in with the biggest components - the Battle Born Lithium batteries and a new Victron MultiPlus inverter / charger . We'll also be laying the groundwork needed for installing the remainder of the system later on in the series too.

RV Electrical Upgrade: Part 2 – Battle Born Lithium Batteries & Victron Inverter

All opinions in this article are entirely mine based on my own experience including using Battle Born batteries for over two years, the data we have collected and the results of the Battle Born study.

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Battle Born Cold Charging Study

The Battle Born study is called the Cold Charging Study, but really focuses more on discharging than charging. The study considers two variables - the discharge current and temperature.

In summary, the results really demonstrate two key points, relating to the two variables:

  1. The usable capacity of lead acid batteries reduces significantly more than that of lithium batteries as the temperature decreases;
  2. The usable capacity of lead acid batteries reduces significantly more than that of lithium batteries as the discharge current increases;

I'll examine each of these in turn, overlaying my own experience where applicable.

A lot of people who ask about the performance of lithium batteries in cold temperatures do so because they have heard that Battle Born batteries cannot be charged below 25°F and cannot be discharged below -4°F. This is true. The built-inBattery Monitoring System (BMS) kicks in to enforce those limits

However, what many people don't appreciate is that lead acid batteries aren't immune to the effects of cold temperatures, and that's exactly what the Battle Born study is attempting to demonstrate.

The study compares two battery configurations: two AGM batteries with a rated capacity of 105Ah each (total capacity 210Ah) and two Battle Born lithium batteries with a rated capacity of 100Ah each (total capacity 200Ah).

However, it's important to realize that lead acid batteries (including AGM batteries) are given a rated capacity at a particular discharge rate (due to the Peukert effect that we'll get onto in the next section). This is typically a 20-hour discharge rate.

Although Battle Born don't disclose in their study or white paper which AGM batteries they used, I found a Group 31 Lifeline AGM battery with a rated capacity of 105Ah at a 20-hour discharge rate. In other words, it has a rated capacity of 105Ah at ~5A, or for two of them in parallel it would be 210Ah at ~10A. For the purposes of this article, I'll assume this is similar to the battery they used for the study.

Temperature: Lithium vs Lead Acid

In their study they tested each battery configuration at a range of discharge currents and temperatures. Starting with temperature, I'll just use their lowest discharge rate for the purposes of comparison.

Even at 30A at room temperature, the AGM battery was only able to deliver 63Ah of usable capacity, where the Battle Born lithium battery delivered 207Ah (even higher than its rated capacity due to manufacturing tolerances to ensure every battery is at least 100Ah capacity).

While their study measures the usable capacity while controlling for temperature, it doesn't include a chart of the results. Maybe it's just the scientist in me, or maybe because I prefer graphical data, but I had to make one - so here it is, based on their data.

Usable capacity of the battery vs temperature as a percentage of its usable capacity at room temperature.

To help isolate the impacts of the high discharge current compared to the AGM's rating, I've normalized the data in the chart above to show usable capacity at different temperatures as a percentage of its capacity at room temperature, all other things being equal.

So what does it show?

Well, put simply, while the lithium battery capacity does reduce, the AGM battery suffers significantly more as the temperature drops. In fact, even attempting to normalize for the Peukert effect on the rated capacity at 30A, the lithium battery retains 80% of its usable capacity at ~15°F, whereas the AGM battery barely manages 50%.

In other words, even at extremely low temperatures, the Battle Born lithium batteries significantly outperformed the AGM batteries in terms of rated capacity.

Peukert Effect: Lithium vs Lead Acid

Why am I making such a big deal of the discharge current? Well, it's down to something called the Peukert effect.

Anyone who has configured a battery monitor such as the Victron BMV-712 will have been asked to input the Peukert exponent, but in case you're not aware of what this means, let me briefly explain.

Essentially, Peukert's Law says that as you increase the discharge rate, you decrease the capacity of a battery. In other words, the capacity of a battery is inversely (and exponentially) proportional to the rate at which you discharge it. The extent to which this happens depends on many factors, including the battery capacity - and is typically quantified by the Peukert exponent.

A perfect battery whose capacity were independent of the discharge current would have a Peukert exponent of 1. In the real world, this value is higher than 1. Battle Born advises setting this value to around 1.05 for their batteries, and lead acid batteries are typically be in the range of 1.1 - 1.3 (according to Wikipedia).

While the Battle Born study doesn't state the Peukert exponent for the AGM battery, we can calculate it using their data. Using the room temperature data, the AGM battery has a Peukert coefficient of 1.9-2.5. This is much higher than expected, so I suspect the 12.2V cut-off used in the study is a little generous.

That said, many owners are taught to not let their AGM batteries drop below 12.2V to avoid potential damage so this may be fairly representative of the real world. Either way, that was the decision made in the study.

Using just the room temperature results at a discharge rate of ~0.4C (discharge current divided by capacity), the Battle Born battery was able to deliver 95% of its rated capacity whereas the AGM managed just 5% of its rated capacity before reaching the 12.2V low voltage cut off.

Peukert Exponent AGM vs Battle Born
As the discharge current increases, the usable capacity decreases - an effect that is far more significant in lead acid batteries than LiFePO4 batteries such as Battle Born's lithium battery.

Regardless of the exact value, there's no debate that the Peukert exponent for AGM batteries is much higher than for lithium batteries due to the lower resistance of the LiFePO4 chemistry. The result is that lithium batteries are able to deliver significantly more capacity than AGM batteries at high discharge rates.

Analysis

When we bought our RV, one of the first things we did was to install three Battle Born lithium batteries as part of our big electrical upgrade.

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We had also heard concerns about the performance of lithium batteries in cold weather, and I'd be lying if I said that wasn't one of the reasons we decided to put them under the bed in our RV. However, temperature concerns aside, we would still have installed them there as it made sense for the configuration of our storage and to keep them secure.

Battle Born's study provides data to refute the hypothesis that lithium batteries are more susceptible to cold weather than lead acid AGM batteries. The data clearly show that lithium batteries can provide significantly more usable capacity before reaching a low voltage cutoff than their AGM counterparts.

Real World Discharge Rates

The study also demonstrates how well lithium batteries fare with high discharge rates, however I would contend that such discharge rates are unusual in the real world for RVers.

We collect a lot of monitoring data in our RV and I ran the numbers (see the Appendix of this blog post for full details) to see how often we discharge our batteries at the rates used in this study.

Looking at 12 months of data between July 2019 and June 2020, we observed the following discharge rates (normalized for our 300Ah battery bank):

Discharge rate% of time spent discharging faster than this
0.15C (45A)1.20%
0.25C (75A)0.88%
0.40C (120A)0.28%

As you can see, these discharge rates are pretty unusual for us!

How come? Well, even ignoring Peukert's Law, your batteries will run out pretty quickly if you discharge them that fast.

To give some context, for us to achieve a discharge rate of 0.4C (120A for our 300Ah of batteries) would generally involve running a high power appliance such as our air conditioning unit, electric water heater or microwave. These aren't things we run for long when running off batteries because, well, our batteries will run out!

When we do run those loads tends to be when we have shore power (or a lot of solar coming in) and in those situations we're not draining the batteries - or at least not as fast.

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Peukert's Law

In the room temperature test, the AGM battery managed just 9.4Ah of usable capacity at 80A. That means it lasted barely 7 minutes before hitting its low voltage cutoff. Forget running the AC or electric water heater, you would completely drain the batteries just running the microwave to heat dinner, right?

Well, not quite.

Peukert's Law specifically applies to a constant current being applied until a cutoff voltage is reached. A higher Peukert exponent doesn't mean the battery mysteriously loses energy, say, due to heat. No, a significant amount of energy remains in the battery - it just can't be extracted at the same, high rate.

In other words, once the high discharge load is removed, the voltage will recover, allowing more energy to be drawn from the battery. It's worth noting that the voltage will take time to recover (hence why it's always recommended to let lead acid batteries rest without any load for a while before measuring the voltage).

So while you might not be able to run your high power appliance for more than a few minutes on the AGM batteries, there will still be plenty of power left for your lights, fans and other lower power devices.

Our Recommendation

Like many, we were put off by the high price of lithium batteries when we first began researching our options. We considered AGM amongst other options, but eventually settled on lithium - and we couldn't be happier! If we had to do it all over again, we'd do exactly the same.

RV Electrical Upgrade: Part 1 - Introduction & Goals

We knew long before we bought our RV that we would want to upgrade the electrical system. Most RVs are designed to be hooked up to shore power (or a generator) whilst being used, and the battery system is usually just to provide a bit of power during travel. As two self-confessed technophiles, living full-time in the RV and harboring aspirations of off-grid living, this wasn't going to work for us.

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While the study is only focused on cold temperature performance and high discharge rates, there are several other benefits to lithium batteries that made it an easy decision for us in the end.

Using the configuration from the study, the lithium batteries have a combined weight of 62lbs, whereas the two AGM batteries are more than double that at 128lbs.

While the AGM batteries are roughly the same size as the lithium batteries (around 1,600 cubic inches), the lithium batteries can be installed in any orientation - including upside down if you wanted! And while AGM batteries don't typically vent in normal operation, they may do for pressure relief - and we didn't want that under our bed. That would have forced us to put the AGM batteries on the tongue - increasing the tongue weight on our RV, something we were trying to avoid.

The Battle Born lithium batteries do cost more up front, but as full-timers, we're confident we'll get our money's worth. Battle Born claims their batteries will still have up to 80% of their original capacity after 3,000-5,000 cycles - that's up to about 10x more than an AGM equivalent. For context, we've cumulatively drawn 73,055Ah from our batteries (with an average discharge of 160Ah) since we installed them - or the equivalent of 243x their capacity.

Summary

I applaud Battle Born for sharing some real data on the performance of their batteries vs AGMs in cold temperatures. While I would contend that the high discharge rates may not be representative of real world behavior for RVers such as ourselves, cold temperatures are a real concern!

Even if you're just a fair-weather camper, just one cold night could be a double whammy for AGMs - cold temperatures pulling down their usable capacity, and higher discharge rates as you run the furnace to stay warm.

For us personally, we're in the midst of preparations to spend this winter in some very cold temperatures in northern Vermont - down to 0°F or even lower! Our Battle Born temperatures will stay nice and warm under our bed - the lowest temperature I've ever recorded them reaching is 52°F.

On the other hand, if we currently had AGM batteries on the tongue of the RV where they'd be exposed to the extreme cold, I'd be pretty worried. While we plan to have full hookups all winter, power outages can (and will) happen. Knowing that we have our batteries to fall back on is a huge relief, and one we appreciate!

So if you've been considering new batteries, and have been considering lithiums but have been put off by concerns about the temperature, this study should give you some peace of mind.

While the lithium batteries are more expensive up front than their lead acid counterparts, if you can afford it, we would highly recommend them. Our Battle Born lithium batteries have been operating flawlessly under our bed for over two years now - what more could you ask for?

Appendix: Data Queries

We capture all the data from our electrical system in InfluxDB, using the MQTT broker built into our Victron CCGX - part of our Smart RV automation system.

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This emits data in near-realtime, with readings for constantly varying metrics such as battery current emitted on average every couple of seconds.

For this blog post, I used a dataset covering the 12 month period from July 2019 to June 2020, which included over 14 million individual readings for battery current alone - or one reading every 2.2 seconds on average.

To keep things simple, I took the median value each minute, essentially giving a resolution of 1 minute - just to reduce the query times. This shouldn't bias to over- or under-estimate and while it may miss short spikes, for the purposes of this article it is sufficient detail.

Charging is reflected as positive current and discharging is negative. To calculate the amount of time spent in excess of each discharging threshold, I counted the minutes where the median current was below said threshold.

During the 12 month time period studied we have coverage for 96.8% of the time (i.e. 96.8% of minutes have at least one current reading).

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