About those batteries

I heard back from the battery dealer, and it looks like we will not be able to pick them up until Wednesday. After checking on the camping situation on the coast, which is slim pickings at this time of year, we decided to just spend another night right here at Lee State Park.

In the meantime, regular reader Rod left a comment on this morning's post that I think merits an answer here in the main text of the blog. Rod writes:

I am certain that customers requiring eight 500 amp-hour AGM batteries don't come along very often for battery vendors. Since the price of this seems to be at least a dollar an amp-hour, I am curious as to the number of cycles that can be expected with your bank. I read that discharged 80 percent, one could only expect 400 cycles. That would be $10 a cycle which seems quite high. Perhaps the trick is to never discharge below 50%. Just curious. ...

First, let me correct the numbers: the Trojans are 230 amp-hours (AH), some of the other batteries we considered were as large as 260 amp-hours (although that seems a bit optimistic for me for an 8D), and they averaged around 245 amp-hours in the 8D size. That makes our bank of eight a total of 920 amp-hours (at 24 volts; remember these are 12-volt batteries). That puts the cost per amp-hour at about $4.24 for our 24-volt system; it would be $2.12 per amp-hour for a 12-volt system.

That's about the going rate right now for AGM batteries. You might find a lower per-AH cost for batteries that are in a more common size (8D is not a very common size for traction batteries), but mostly, batteries of all types are a commodity item and the price fluctuates broadly based on supply and demand. Cost of materials does also enter into it, and lead and other components of these types of batteries, also commodities, have been rising steadily over the past few years. By way of illustration, most vendors wanted to charge me about $60 more per battery without a trade-in, so my used, fully depleted 8Ds are worth that much just in recycling value.

Now, to answer the question, the number of cycles, as you note, depends heavily on the depth of discharge (DoD). Some reputable manufacturers actually publish a chart or graph plotting cycle life vs. DoD. For example, the chart on the last page of this document, from Trojan, shows that Trojan AGM batteries can deliver nearly 2,000 cycles at 40% DoD, dropping to 1,000 at 55%, 450 at 80%, and fewer than 300 cycles at 100% DoD. Charts from other manufacturers are similar, and one can extend the summary results of these charts really to most brands of AGM battery in the same size range.

Armed with this information, it is possible to do some optimization of discharge/charge cycles to maximize the value, rather than the lifetime, of the batteries. For example, while phenomenal cycle life is available by keeping to within only, say, 25% DoD, the fact is that putting the last 25% of energy back into the batteries takes way more power than putting the first, second, or even third 25% of the energy into them. Lead-acid batteries, like many other things in engineering, follow what we like to call the "80/20 rule," wherein putting the first 80% of charge into the battery takes 20% of the power, and getting that last 20% into it takes the other 80%. Any given battery will not be those exact percentages, but you get the idea.

For this reason, most of us put that last 20% in, a process called "topping off," only when the cost of power is very low. Typically, this is when we are on shore power, provided as a fixed part of the cost of a camp site, for example. Many of us choose to stop the charging process at about 80% state-of-charge (SoC), or in other words still at 20% DoD, when charging from a costly source such as a diesel generator, where the cost to run can be upwards of $5 per hour.

If you know the number of cycles vs. DoD, from a chart such as Trojan's, and the charge absorption profile of the battery, and the capability of your charger, and the cost per kilowatt-hour (kWh) to supply that charger, you can completely optimize the exact DoD to be routinely using in order to minimize the "cost per stored kWh" for a given set of batteries.

That's all well and good, but for RVers in the real world it's not that simple. That's because our discharge needs, charging capabilities, and cost per recharge are highly variable. Ultimately, we end up with a different DoD for almost every cycle. Even boondocking for two weeks in the desert, where you'd have the most control over when to start and stop the charge process in each cycle, the fact that most generator auto-start systems can only work on voltage and not actual DoD will mean that you are not controlling the process precisely enough.

I'd love to tell you how many cycles we got out of this set of Trojans, but I simply can't. Most battery monitors, ours included, simply can not keep enough history. What I can tell you is that our meter counts up to 1,999 cycles, and we're well beyond that. However, it also shows that our "average" DoD is a mere 20 amp-hours. That's because 20 amp-hours is deep enough to cause the meter to start counting, and so even if we stop the bus for an hour to get a bite to eat, or go shopping, or whatever, we'll get a 20 amp-hour cycle, as the main alternator will very quickly charge that amount back up while we drive.

Even as we sit here today, connected to a 30-amp pedestal, we are racking up 20 AH cycles. We have our inverter max input dialed down to 24 amps, but when two air conditioners cycle on at the same time, we are drawing 26-28 amps, plus whatever else we are using, and the inverter supplements it from the batteries. That can draw the batteries down 20 AH before an A/C cycles off, dropping the load below 24 amps and causing the charger to start recharging the batteries, and the cycle repeats ad infinitum.

This same meter also tells me that my greatest DoD since we installed these batteries was 877 amp-hours, or a whopping 95% of capacity. (Shortly after that episode I adjusted the LBCO on the inverter to prevent this from happening again.) In practice I can tell you that when we are boondocking for multiple days, we draw the bank down to about 75% DoD and charge it back up to about 75% SoC, so we are using about 50% of the total capacity, and this happens over and over again, with a cycle time of about two days in temperate weather, or a mere half day (12 hours) if we are running an air conditioner.

When we drive every day, in temperate weather, the generator never runs, and we drop to about 20% DoD while parked, and recharge to 100% SoC while driving. Without a complete history of the last four years, I can't say with conviction how many of our cycles are the 75%/25% variety, how many are the 20%/0% variety, and how many are the 10%/0% variety that the meter likes to count.

Doing some back-of-the-envelope math, though, I can make some guesses. Completely ignoring the 10% "blips" that the meter likes so much to count, I would guess conservatively we have over 2,000 cycles on these batteries. I would further guess the average net DoD of those cycles to be in the 35% range, or right between the 20% sort when we drive and the 50% sort when we boondock. That's consistent with Trojan's life cycle prediction for these, which shows about 2,500 cycles at 35% DoD.

If my guesses are correct, and adding in some of those 10% cycles for good measure, we've gotten (and replaced) about 650,000 amp-hours, or over 15,000 kWh from this set of batteries. That works out to a cost of about $0.25 to "store" a single kWh of power. This is in addition to anything it might cost to generate that power, and might be compared to the nationwide average cost of grid power, about half that amount. So on an RV, even "free" electricity (say, from solar, or included in campsite fees), stored for later use, costs more than grid power at a typical fixed structure.

That's a very long-winded answer to your question, but it is a source of much confusion and a topic on which I often spend a good deal of time in my seminars. This is one of the reasons I often have to caution people with unrealistic expectations about "free" solar power, or installing large battery plants in preference to an appropriately-sized generator. There are many good reasons to have a large battery bank, but cost savings is not a slam-dunk; it varies widely by circumstance and you really need to do the math.

This provides a good opportunity to reflect on our current mode of travel, "power pole to power pole." As long-time readers know, we don't generally do this and it is not our preferred way to live. Mostly it is something we do when we are traveling through hot or humid environments and we need air conditioning full-time, a sort of travel we generally avoid unless we have a specific destination, such as a Red Cross deployment, or a conference someplace, where we need to be.

For many RVers, though, this is standard practice, and while it is not our own preference, it makes perfect economic sense. This is especially true for those who are not full-timers, and who use their rigs for a couple dozen nights each year. You can buy a lot of $30-$50 camp sites for what a good set of batteries will cost, and when you're done camping, that camp site investment does not require any ongoing maintenance. This, too, is something I discuss in the seminar.

When all is said and done, there is not a single one-size-fits-all answer for the right balance among batteries, inverters, generators, solar, and power-pole usage for all RVers. Our set of choices is almost the right balance for us. If I were in the market for a generator today, I would buy a 6.5 kW as opposed to the 15+ kW unit we have now (and which came with the bus). But I would definitely not trade in my large battery bank, and the flexibility it buys us, even though it costs us about a grand a year.

Photo by mikeysklar, used under a Creative Commons license.