I don&#;t know about anyone else but I&#;m getting bored with lithium batteries&#;but that highlights one of the biggest pitfalls awaiting a cruiser planning to install them:
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There is so much to know that it&#;s tempting to pull out the credit card before completing the planning process properly.
I freely admit I have done exactly that with other projects but it never goes as well as when I grit my teeth, finish the research and then complete the plan, all before spending the first dollar.
So let&#;s soldier on. At least this one is a relatively short read. Brace yourself for when we get into the more complex issue of the level of DIY each of us may wish to take on in the next and hopefully last article in this buyer&#;s guide, before we end with the payoff: a simple buyer&#;s checklist.
With that blather out of the way, let&#;s quickly cover the fundamentals of fusing.
In the last chapter I explored why we offshore voyaging sailors need a battery management system (BMS) that can intelligently control charging sources rather than panicking and shutting down the whole works&#;totally unacceptable on an offshore voyaging boat.
If you still don&#;t agree with that after reading the last chapter and the above linked one, that&#;s fine with me, but let me be clear that as far as I&#;m concerned, this is a fundamental requirement for a seamanlike lithium-based electrical system, and everything that follows is based on that.
Given that, I will answer any comments that argue with this premise with &#;fine, let&#;s agree to disagree&#;, rather than get into it yet again&#;that&#;s as nice a way as I can come up with of saying &#;you&#;re welcome to your opinions but please respect mine&#;.
That said, if you have batteries with a BMS that does not communicate, or that&#;s the route you want to go&#;&#;drop in&#; batteries with an internal BMS that does not have process control&#;I will cover how to do that in a seamanlike way in a later chapter, so let&#;s not go there now, other than to say that such a system will never be as good as one done right&#;in my opinion.
Before we dig into the simple wire or CAN Bus decision, which I will now cover in a later chapter, let&#;s explore two other buying criteria that either type should satisfy:
Let&#;s start with cell balancing, because understanding that informs our decision on monitoring&#;this is sure to get everyone warmed up.
First off, we need a BMS that balances properly and automatically, and a system set up to let it.
Who says? I do&#;oh, not good enough? You&#;re right. How about this? Al Thomason at WakeSpeed says.
I spent over an hour on the with Al before writing this article and most of that was on balancing and making that happen.
Al has worked with most of the major lithium-battery and BMS manufacturers over some five years to develop profiles that will charge their batteries as efficiently as possible, while not shortening their lives by over-charging.
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No one, but no one, and certainly not that handsome silver-tongued presenter on YouTube who purports to make all this simple, or that guy on the forum who has all the answers with absolute certainty, has even a fraction of Al&#;s understanding.
Here are the key things Al told me, and other credible sources confirm:
All lithium batteries, no matter the quality, tend to drift out of balance over time.
For the purposes of this article I will assume that the battery was fully charged and properly top balanced when new, and focus on ongoing balancing while in service.
If you are interested in the initial process, the Victron Lithium Smart Battery Manual has a good explanation (section 4.3) of what needs to be done before new batteries are put into service.
One of the biggest myths around lithium batteries is that if we never charge them above around 13.5 volts (or close) we don&#;t need a BMS that controls charging sources since we will never be close to the BMS cut-off voltage of ~14.4 volts.
Yes, that&#;s true&#;for a while, but the problem is that at that voltage the battery won&#;t balance and so will drift over time.
How long before problems show up? That depends on usage and how well balanced the cells were in the first place, how the BMS works, the owner&#;s astral sign, and on it goes. Could be years, could be months. More likely the latter than the former.
So, yes, just charging at low voltage will work, but it&#;s far from best practice, and on an offshore voyaging boat only best practice will do, particularly since it&#;s not that hard or expensive to do it right.
Keep in mind, it only takes one weaker or stronger cell to start causing blackouts if not properly managed.
No, Al and I didn&#;t make this up. This is just one example of the instructions from a battery manufacturer:
We recommend a minimum absorption time of 2 hours per month for lightly cycled systems, such as backup or UPS applications and 4 to 8 hours per month for more heavily cycled (off-grid or ESS) systems. This allows the balancer enough time to properly balance the cells.
Victron Smart Battery Manual
Wait, it gets worse. Lithium cell balance drift is usage dependant.
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