size of lithium battery bank - 3.2v cells

Scroll down to the pros and cons on this page:
LiFePO4 Batteries On Boats https://marinehowto.com/lifepo4-batteries-on-boats/

Perhaps I'm misinterpreting what he says here, but there's lots of others who have researched these batteries making the same claims. But I personally have no experience.

 

Most lead acid banks are planned for up to 50% DOD, but Lithium banks can handle deeper discharges. It's amazing you haven't seen this before during your research - it seems to pop up all the time.
Mark

 

That number pops up frequently. However, rules of thumb are based on proven facts or technology. Lead/acid and Lithium Ion batteries (actually a hybrid energy storage device) are families and can't be compared without defining which type of chemistry you are referring to.

 

The English phrase rule of thumb refers to a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It refers to an easily learned and easily applied procedure or standard, based on practical experience rather than theory. This usage of the phrase can be traced back to the seventeenth century, and has been associated with various trades where quantities were measured by comparison to the width or length of a thumb.

Rule of thumb - Wikipedia https://en.wikipedia.org/wiki/Rule_of_thumb

Seems fitting - Mark

 

For a rule to have broad application, it has to generate good practical results. I keep on hearing about depth of discharge (DOD) but without any quantification. We test batteries to 10V discharge and use, among others, Peukert's equation. It gives a good comparison between batteries. How do you quantify 50% discharge?

 

Do you not have google ?

Depth of discharge - Wikipedia https://en.wikipedia.org/wiki/Depth_of_discharge

 

Ford or Chevy?

I belong to both groups and enjoy reading and sharing. IMO if the idea is to draw a line in the sand and argue well have at it.
On the other hand we could join the general population and respectfully enjoy each other and the different views.
IMO lead is good if used properly.
IMO Lithium is good if used properly.
IMO the success is in the design and application of each type.
Another person on the Lithium side pointed out not too long ago it is important to make sure your insurance company will cover the boat if you build your own system. I won't use her name because I don't have her permission. That said, IMO very sage advice.

Blessing and fair winds always,
Brian

 

Hi Brain, I am just replying to questions asked. Sure my last post was a little snarky, but the line of questioning seems like something that is public knowledge.
Cheers,
Mark

 

While most definitions that apply to batteries are common knowledge, their implication relative to practical use or utility of any particular battery chemistry is not always intuitive. For instance...

First, when someone says 'lithium', it is usually shorthand for 'lithium-iron-phosphate' (LiFePO4) which is the most common and affordable lithium battery chemistry. More exotic lithium chemistries exist, but marine applications are exceedingly rare. Unless you're hoping to power your boat with a used Tesla battery pack, a 'lithium-nickel-cobalt-aluminum' (LiNiCoAlO2) discussion is not helpful. And also a poor choice on many levels.

On the other hand, 'lead-acid' is certainly not specific enough to have an intelligent discussion. While minor variations around cathode coatings exist, flooded (FLA), glass mat (AGM), and gel (GEL) all share the same chemistry, albeit in different physical configurations. Each has different operating characteristics, so specifity is required.

The most challenging definition, in my experience, is 'depth-of-discharge' (DoD). This is simply is the percentage of rated battery capacity that you consume in a discharge 'cycle'. And a 'cycle' is variable period of time between when a fully charged battery begins to discharge current, until the subsequent point when said battery is (fully) recharged.

The challenge with DoD and cycle is that these two parameters are highly related in ANY battery, regardless of chemistry. The greater the average DoD, the fewer number of cycles the battery can be recharged before it needs to be replaced. Fortunately, every reputable battery manufacturer publishes DoD vs cycle data in a easy to read chart. Unfortunately, this has led to some interpretations around battery performance that are either misleading or inapplicable in practical use. The 2x capacity rule of 'lead-acid' vs 'lithium' being the most egregious.

Consider a 100Ah FLA, AGM, of GEL battery, rated at 1000 cycles for 50% DoD, fairly typical. If held to only 20% DoD, this same battery may last 1500 cycles. Conversely, if DoD is routinely and significantly greater than 50% DoD, the same battery may only last 400 cycles. In the case of FLA, discharge significantly greater than 50% can have you back at the chandlery in a matter of months! Important to note that AGM and GEL can run down to 80% DoD occasionally without a huge loss in total cycles. This data is widely available, so simply look it up once you have an overall energy budget and decide what chemistry and capacity is right for you.

Other than safety and simplicity, lithium has a clear advantage in both DoD and cycles, typically 90% DoD and 2000+ cycles. However, as noted in a previous post, the 80/20 rule applies in the real world. Most of us will never burn though 2000 cycles in our boats before we move on to the next boat (or no boat!). And the economics per Kwh are undeniable...AGM is twice as expensive as FLA, and lithium is twice as expensive as AGM.

In summary, for a 100Ah battery you may get 90Ah of 'usable' capacity in lithium, compared to 50Ah in lead-acid, and you'll get alot more cycles (and peak discharge current) from lithium. But after factoring in a safety margin for emergency capacity, maybe 50-100%, and taking an clear-eyed look at your energy budget and cruising behavior, it's unlikely that those 40Ah are going to be a deal-breaker. But you needn't take my word for it. Between published data and knowing your own boating behavior, you can come up with a good decision if you take the time to think it through.

 

Thanks It's all good!

 

: >

 

I do have Google, and know about Wikipedia which is often full of nonsense. It does not give any useful quantification of charge. The power available from a battery depends on the rate of discharge. Peukert's equation is one way of comparing different batteries, or the same battery at different modes of operation. You have not defined what 50% charge means. If the Li Ion battery can discharge to 3 V without damage, it is meaningless since electronics will shut off at 9.5 V. That is the reason that the industry standard is to measure discharge to 10 V. Further, you have not explained how you measure the power output to be able to compare them.

 

I have to disagree. Flooded cell and AGM have a marked difference in chemistry. Particularly, the intermediate reactions are different. For example, in AGMs, there is a recombination of H2 and O2 into water. In vented flooded cell batteries the gasses are lost necessitating the routine addition of water.

 

Fair enough, Gonzo. Hydrogen reabsorption is a big deal with AGM with real utility in terms of safety below deck and maintenance, as you say.

For that matter Gel batteries are similar but also quite different in terms of chemistry. Aside from being very particular about charging cycles, however, their utility is questionable for the price IMO.

I guess it’s fair that lead-acid types are generally similar, relative to lithium types. But generalization is always dangerous, which is my point (generally)