MLW sub $500 robot

Well, I'm all about "why." I've done a bit of research on battery technology over the years, and if you know something I have not come across I'd like to read it. I just have a quark about hear-say science, like "you are safe in a car, during a lightning storm, because of the rubber tires*." I'm sure the batteries I have are deep cycle according to what I know, and I reject the notion they they are not without any supporting information that can explain why.
*
And yes, I know it is *NOT* because of the tires. I almost shot my TV when they said this widely believed and obviously wrong nonesense on CSI.
Reply to
mlw
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Hmmm. I'm running a Pentium M (2 GHz) and sensors for 5 hours on 24V at 7 amp-hours.
I could get a 6 amp-hour LiPol 30V at 6 amp-hours for about $500. A 7.2V 6 amp-hour for my servos is around $150.
And this is at Robot Marketplace prices. -- D. Jay Newman
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Reply to
D. Jay Newman
Because UPS and fire-alarm batteries are made to be continiously charged. Deep cycle batteries are more expensive and therefore are typically not used for these applications.
This is just from what I have gathered over the years.
Maybe I'm wrong.
Reply to
D. Jay Newman
My problem is not running the computer or sensors, it is running the dc motors. What do you use in that case? I need 20 amp hour just for mobility. (bear in mind it is an outdoors rover supposed to go over rough terrain at relatively high speeds)
Reply to
Padu
NiMHs or SLAs are probably best for that unless you have government funding. :) -- D. Jay Newman
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Reply to
D. Jay Newman
OK. From my experience with UPSs, the batteries die after only a few times of deep useaga (power failure with nobody present to turn things off so the cells were drained).
Heck, I've seen a UPS killed because their usage was just a bit above the charging rate (one of our artists got a bigger monitor than was planned for).
Replacing the batteries fixed the first case, and replacing the UPS with a bigger one fixed the second.
I haven't had experience with motorcycles in years, but if they're like car batteries, they aren't the same if they've been drained even once.
From what I know of business, extra features cost extra money. If the normal useage doesn't need it, then they'll drop it.
Deep cycle useage makes a battery that isn't as good at being a motorcycle or UPS battery (where constant charge and complete draining means other badness). And it costs more to make a battery that is both deep cycle and has low resistance.
On the other hand, wheelchair batteries are meant to be driven into the ground.
Reply to
D. Jay Newman
This exactly mirrors my experience. I have no reason to believe that all SLA batteries for a UPS are deep cycle, or that all SLAs are deep cycle.. UPS's are a competitive product. I'm not sure they'd always use the more expensive deep cycle construction for a device that only rarely requires discharge?
This isn't to say some of the better UPS's may not use deep cycle batteries. I'm sure the commercial and industrial ones do. The consumer APCs that I've hacked certainly didn't. They failed to keep a charge after about a dozen full cycles. It seems reasonable that to keep costs down they used thinner plates. Thinner plates=less lead=less $$$.
SLAs for solar backup, however, are made for deep cycle. They're more expensive, but I've used a 12volt AGM battery for about two years and deep charged it dozens upon dozens of times, and it still holds a charge like it did when new. Far better than the motorcycle batteries I used to use. For one thing, they periodically dried out.
-- Gordon
Reply to
Gordon McComb
UPS and fire alarm applications are a "very" bad situation from a battery's perspective. Being on a charger 24x7 will kill any battery.
Like I said, UPS systems constantly charge the battery, even the systems that periodically test the battery and apply a charge when needed will kill a battery in a couple years because of over charging.
I don't recall if bikes have deep cycle or not.
I'm not sure that "Deep Cycle" batteries are more expensive to make. They generally have fewer and thicker plates. I can see a "cheap" battery with thinner plates, but it would not take long for the various testing houses (UL, CA, etc.) in the fire alarm business to rate these as poor replacements.
My cousin partially owns a fire alarm company and of the things that real estate holding companies skimp on, fire alarms aren't one of them. No cost is too high compared to insurance premiums.
Few lead-acid batteries will survive complete draining, a good deep cycle battery will typically survive discharge to 20% of capacity.
A deep cycle battery that has low resistance is expensive, true. To be deep cycle, it need to have thicker plates so they don't warp or disintegrate right away.
There are a lot of different types of lead acid batteries and a lot of different depths to "deep cycle." For instance, an open non-maintenence free battery with caps that use lead-antimony for very strong plates and can thus be discharged deeper than a sealed and/or gel battery with lead-calcium plates which are chemically more stable and will have longer life.
What, I think we are talking about is "battery life," i.e. how much discharge can it have and how many, and how long just sitting on a shelf. Lead acid batteries don't have "memory" which is good, but suffer a great deal of damage on deep discharge.
Gel type deep cycle batteries will fail sooner than AGM batteries (AGM absorbed glass material, sulfuric acid soaked fiberglass), pure lead/liquid batteries typically out last both.
AGM and liquid batteries are a problem in a hobby robot that may tip, so sealed gel type is what is called for. Since I am using a gel battery, I have to expect about 2-4 years life and 40%-50% of charge cycle.
Reply to
mlw
As I have posted to Jay, there are many depths to "deep cycle." The internal construction of the battery defines the depth of deep cycle.
The are differences in the electrolite, AGM, gel, and liquid sulfuric acid, there are differences in plate construction, plate thicknesses and whether or not they are solid, etc. "Deep cycle" is applied to batteries that are neither "starting" nor "marine." Starting batteries should not be discharged more than about 10%, marine no more than about 50%, and deep cycle no more than about 80% of capacity.
Some deep cycle batteries may be able to survive getting cycled lower than 20%, but they cost a lot more.
Also, you can extend battery life a lot by not deeply discharging the battery. If you get a larger battery and only discharge it to 50% of capacity, it may last (if I remember correctly) twice as long.
Reply to
mlw
Why? There are a lot of different types of lead-acid batteries and they provide different resistences to deep cycle damage.
When ever there is current flowing through a battery there is a chemical reaction happening, there is *always* some amount of damage that happens to the battery. Some less damging than others. Within normal charge range, the battery suffers very little damage. During overcharge or deep discharge the battery suffers greater damage.
It is hard to make a battery charger that does not over charge the battery or cycle the battery a lot. It just is. Battery charge decays with time. A UPS tries to to keep it at 100% charge. The battery is constantly discharging by some small percent (with decay and voltage monitor current drain) and being recharged to full when the tricle charger detects the voltage drops below a certain point. This is a small discharge/charge cycle, and over time they add up, more so than charging and disconnecting it and keeping it on a shelf to be charged next week.
Still, a UPS battery will last a year or two, maybe even three and be reliable.
Reply to
mlw
As I posted in another thread, this isn't really true. A battery will naturally discharge over time. Constantly charging/discharging +- 1% will add up over time, and a battery left on a shelf and tricle charged one night a month will probably outlast a battery left on the charger 24x7
Reply to
mlw
For such a pedant (e.g., "There are a lot of different types of lead acid batteries and a lot of different depths to 'deep cycle.' "), this is sort of a ridiculous thing to say, isn't it? At the limit, you could have a human being in the loop as part of the "charger". He would decide when the battery should be charged using whatever optimum algorithm there might be. If being on that "charger" would kill any battery, what wouldn't? Now just replace the person with a smart charger.
And why is trickle charging such a bad situation from the perspective of a lead acid battery?
Mitch
Reply to
Mitch Berkson
Just a nit, but this depends on the charger. Being on a _crappy_ charger 24/7 will kill a battery. A battery can be left pretty much indefinitely on a charger properly designed for the battery chemistry. Invariably, a good charger is usually worth the money to buy or the time to build.
Reply to
The Artist Formerly Known as K
They are probably as near optimum as can be given what they are doing.
It is the purpose, if you expect 100% charge, you expect 100% charge. Keeping a battery at 100% charge has costs.
Reply to
mlw
A "reference" that supports exactly "this," no, it is my own conclusions based on the science of chemical energy storage and the data I have seen.
Fact: All charge/discharge cycles of a battery damage the battery to some extent.
Fact: The number of charge/discharge cycles a battery takes has an additive effect on battery life. The deeper the discharge the greater the damage, but number of cycles have an affect as well.
Fact: Lead-calcium acid batteries are a more chemically stable, providing longer shelf life.
Fact: Plates in gel or ACM batteries are typically lead-cacium plates.
Assumption: the battery is not over charged or discharged beyond some lowest point
Conjecture: reduce the number of charge/discharge cycles a battery has, reduce the amount of damage.
Supporting evidence: Batteries last longer in storge than when in service in a UPS or fire alarm. This indicates that charging, no matter how well designed has a negative affect on the battery.
There are tons of books that describe lead acid batteries, and batteries in general. You can even google for white papers.
Reply to
mlw
In keeping with your skepticism of hearsay, I'd be interested if you could provide a reference which supports this.
Mitch
Reply to
Mitch Berkson
There are dozens of battery charging chips available which claim to use near-optimal charging algorithms. Do you suspect that the offerings of, for example, Maxim or TI are inaccurately described?
Assuming that there is some problem with trickle charging, why is it hard to make a charger that charges a battery once a week? Prohibitive with Linux maybe, but it just might be doable with a microcontroller.
Mitch
Reply to
Mitch Berkson
As would I, since this directly contradicts my experience with SLAs and decent chargers.
Reply to
The Artist Formerly Known as K
First, I'm not even sure I understand what you're comparing. A battery which is trickle charged is typically used in a standby application like a UPS. So the useful metric for this application is the number of years the battery can be maintained with the trickle charge and still be called upon to provide its rated discharge.
The useful life of a battery which is cyclically charged and discharged is measured in number of cycles - a metric which is inappropriate for a trickle charged battery.
Any data that you choose will show that deeper charge discharge cycles result in a smaller number of total cycles which the battery can provide. Trickle charging is close to zero depth of discharge. This is, at the least, suggestive.
Panasonic provides information about its lead acid batteries here:
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8 is particularly relevant.
Mitch
Reply to
Mitch Berkson
So you are saying a Gel, SLA (AGM) or simple lead acid will last longer on a a good trickle charger 24x7 that it will being trickle charged once a month?
All lead acid batteries degrade over time and have a finite life no matter how they are handled, however, there are methods of handling that will make them last longer. I think we agree on this.
So, I think we have the basis of a bet.
We need to acquire 2 identical SLA (gel or otherwise) made in the same batch. Two identical tricle chargers. One connected to one battery 24x7, and the other connected to the other battery one 24 hour period every calendar month. At the end of a year, which of the two batteries will have the greater capacity?
Are you up for the bet? I am, to me its a no brainer, I'll take the battery charged once a month.
Reply to
mlw

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