Re: Which Battery to Buy?
 

Stupidity and poor guidance/supervision are very different things (although their consequences can be equally devastating).

Re: Which Battery to Buy?
 

OK,
I think we need a battery expert to step up here but in the absence of that knowledge, I spent a couple of hours researching this over the weekend. Some of the things that are needed here are a battery that will not self destruct when mishandled or when max discharge currents are exceeded. I went looking for batteries with the same characteristics as our current battery. So starting a search with 12 volt and 18 AH and Lithium I came upon a wide variety of battery types and specifications that actually fit in the same size as our current battery. What I found is this. Very few of the batteries listed as "starter" batteries give any specification sheets. On one site I was actually denied access to the linked file. What I was not able to find was exact specifications as to internal resistance or to AC conductance for any Li battery in this class. What I backed into was internal resistance of typically 6-14 mohm per cell for most of the Li types. While the cell voltage is higher in Li (and therefore less cells are needed), that still amounts to more than twice the 11 mohm total for our batteries. A peek at peak current also limits Li batteries in this class to less than 200 amps, about a third of what our SLA batteries are rated. In some batteries there is actually a internal electronic breaker to shutdown the battery when the current exceeds ratings, to prevent catastrophic damage. Most manufacturers warn that exceeding max current leads to thermal runaway. That is where the temperature of the battery skyrockets in an uncontrolled fashion and sets itself on fire. Yes, the Li batteries in this class have better labels, stating they are 20 AH or higher.
Yes they can easily deliver currents at 3C and maintain cell voltage almost until they are depleted, but we do not ask batteries to do that. Yes, they are half the weight, but we don't weigh the batteries. Yes they can deliver impressive currents, but please remember that CIM motors stall at 131 amps. While most teams cannot effectively design a robot to meet the needed series wire resistance to deliver that, it is not uncommon for them to produce more than 100 amps per motor. How many of you use 4 CIM drives or even more? As to the linked Li-sulphur, please note that in the article it states that the sulphur will breakdown within a few cycles rendering the battery useless.
So when looking at all of the factors, weight, max current, reliability, cost, and ability to withstand most abuse, the AGM SLA we use seems to be the best of all worlds.

Re: Which Battery to Buy?
 

I design battery systems. That knowledge is in the thread already ;)

Most of the above you looked at is likely NOT LiFePO4. LiFePO4 handles more total cycles, and isn't nearly as damaged by deep discharge as SLA is. These will lead to some big savings and higher reliability for teams.

A LiFePO4 pack could be made for $200 that would be the functional replacement of 2-4 SLA batteries, and be able to source the current required.

Re: Which Battery to Buy?
 

Adam,
Could you link some spec sheets? I am interested in the series resistance specifications and peak currents and basically anything else you can pass along.

Re: Which Battery to Buy?
 

I would want to use pouch based cells for price/space reasons, but a lot of those aren't public facing in terms of spec. You need to talk directly to manufacturer.

In terms of raw chemistry, these are the same as many top LiFePO4 and will perform very similar.
http://www.k2battery.com/products-26650P.html.

It's not uncommon to see 50C ratings as the burst discharge.

It's also not uncommon to see burst at 10C or so for the same chemistry (or have a protection circuit that artifically limits the burst rating, which isn't necessary for FRC). Obviously these cells/packs wouldn't be what we use.

EDIT: Here is spec for pouch A123 cell. http://www.raceyard.de/tl_files/News...Data-Sheet.pdf
This one is interesting in that it also specs all the safety tests it passed.

Re: Which Battery to Buy?
 

A123 batteries is actually where I started my battery research. However, I was not able to find enough information to validate what they had in their datasheets and what they sold.

Re: Which Battery to Buy?
 

Here's a battery I was looking at the other day:

http://shoraipower.com/lfx18l1-bs12-p94

Smaller in size, less than 2.5 lbs, 270 CCA, but say they are conservative and this is closer to 405 CCA in lead acid ratings according to thier FAQ. Never heard of the company before, but thier batteries have good user reviews on amazon. I asked for more info, I'll let you know if I hear anything back.

Re: Which Battery to Buy?
 

I wouldn't recommend them. I bought one to check it out.

Only after purchasing did I realize when they say Pb Eq AH, they mean 3x actual capacity. Discharge test showed that they actually only hit about 25% of advertised capacity (versus 33%).

So, their 18 Ah batteries are actually 6. This is done to account for the approximate 3X+ derating lead batteries get at high current discharge.

They are packaged nice, but a bit spendy.

Re: Which Battery to Buy?
 

Their website is another exercise in frustration. Few Li manufacturers actually publish helpful info. CCA is not a good indicator for us. We need hard facts like internal resistance, discharge curves and other hard data.

Re: Which Battery to Buy?
 

Here is Shora's logic for how they rate their battery compared to a SLA. Their market seems to be drop in replacement for motor vehicle batteries. Motor vehicles (older ones especially) have very simplistic charging strategies.
From Shora FAQ

Re: Which Battery to Buy?
 

Ya gotta love marketing people.

Re: Which Battery to Buy?
 

I agree with Al quite the marketing spin. Lead acid battery mfgs use CCA to indicate the cranking ability of their starter batteries and reserve capacity to indicate how long they can deliver power at a moderate amperage. You won't find an Ah rating on an automotive starter battery.

Re: Which Battery to Buy?
 

Drifting off the batteries for FRC subject... Considering the Shora battery is 2-3 times the cost of an equivalent Lead Acid battery it replaces. The low self discharge is appealing for vehicles that sit for extended periods. Like collector cars, motorcycles, boats, ect. But to get a consistent full charge on the battery you still need an external charger. So you might as well use a battery tender with a lead acid battery.

The advantages Shora claims, while written in marketise is consistent with what the mfr. of our tee shirt cannon Li-Fe battery told us & bears out with use. The Li-FE battery is a lot smaller & lighter than the deep cycle lead acid battery I would have used in its place.

Re: Which Battery to Buy?
 

Shorai batteries aren't who we should be looking at.

If we get an equivalent LiFePO4 battery it will be true 15-20 Ah (versus the 4-6 the SLA ones ACTUALLY are), and will last for many matches. It will also not get wrecked from deep discharge, or from sitting discharged for a while. This combined with a much higher number of total cycles it can do means they would quickly pay themselves off.

We go through 10+ batteries a year due to how much we practice. That number would be greatly reduced.

Re: Which Battery to Buy?
 

Cell manufacturers will provide all this if prompted.