5 ms·

That's interesting! Is it like an SSD where you plan for a certain amount of cells to fail or does it reduce load on the cells so they last long individually?

by foota 8y ago

TheSpiceIsLife 8y ago
Generally speaking, lead acid batteries, and lithium batteries, last longer if they are never (rarely?) deeply discharged. If you need 100Ah from your batteries you’re better off provisioning as many times more up to some cost-benefit analysis.
derefr 8y ago
The latter. Whenever your battery gets super discharged, picture this happening inside it: https://www.youtube.com/watch?v=r-YbQN_twpw https://www.youtube.com/watch?v=r-YbQN_twpw Notice how running the current the other way will break up the formed crystals, but it won't actually re-dissolve them. That's what it means for a battery to wear out. (Battery electrolytes are chosen specifically to be resistant to this, so the effect is mainly visible as a thin "rust" of deposited crystals on the battery's anode, rather than the full-scale crystals seen here.) A similar but distinct process occurs when overcharging a battery, where, instead of splitting the electrolyte molecules apart, you're reacting and bonding them together to form new molecules or molecular complexes, with the reaction usually being one-way rather than an equilibrium reaction. (For lithium batteries this process is exothermic and catalyzed by the presence of the product—thus lithium battery explosions.) It's much better for the life of the battery if it basically just hovers around 40-60% charge for its whole useful life, since then you're just generating tiny seed crystals (on discharge) and then re-dissolving them (on charge), where those crystals are small enough that they can be fully re-absorbed. And this is true even in battery-cell technologies that require a "deep charge cycle" to erase their "battery memory." Battery memory is basically the electrolyte causing enough crystalline rust specifically on the anode to increase its resistance. A deep discharge can capture and erase this rust—but it still shortens the battery's lifespan, because you're still producing non-reabsorbable large crystals within the electrolyte.
- foota 8y ago
  Thank a lot for sharing! That was interesting. So the desire to hover around 50% requires 2X larger batteries as would otherwise be necessary?
  - mikestew 8y ago
    Yup. Though I've heard (as it concerns deep-cycle batteries) not to let them drop below 50% charge rather than keep them in a certain range. So, for example, when I purchased a new house battery for our RV, I got a 200ah battery knowing I can only make use of 100ah before recharging. Or put another way, figure out how much you'll use between charges (or in the case of solar, how much to tide you over until the sun comes up), and double it.
zaarn 8y ago
Not quite. Planning for battery failures is hard because they have different failure modes. For example, the battery could fail to dead short, which is the functional equivalent of an cell in the SSD dying and then setting the SSD on fire. Or the cell might just actually catch fire during normal operation if you overload it. To my knowledge, the best and safest battery system is to have each battery individually monitored by a charge controller with the capability to fully disconnect the battery at will. The downside is that this will be expensive so people usually settle for just eating the rare chance of a dead short battery. Additionally, batteries that you just leave around doing nothing will probably die at some point too. You don't want it sitting around you want to use it for efficiency or else you swap your battery and find out it was dead too. To reduce burst loads on batteries you could use supercaps if you find ones that can handle the voltage and current (that will be very expensive).
- derefr 8y ago
  > Additionally, batteries that you just leave around doing nothing will probably die at some point too. This wouldn't happen if batteries were designed to act like nuclear reactors, where the "fuel" (the fuel rods; the electrolyte) can be completely removed from the substrate that makes it react (the neutron medium; the anode+cathode.) But a battery that can withdraw its anode+cathode from the solution would be damned expensive. It'd make more sense as an architecture if you had just a few, super-large battery cells, e.g. giant vats of lead-acid. It might be possible to design regular battery cells such that they wouldn't start degrading until they were first exposed to a voltage load, though. (I think the "50 year" Duracell NiCd batteries have this property—they probably have an antifuse oxide layer between the anode/cathode and the electrolyte, that gets broken down when you put load on the circuit.)
  - zaarn 8y ago
    That reminds me of a recent NASA project where they kinda built small nuclear fission batteries with less than a couple tons of weight (IIRC down to 100kg) These would be kinda neat as off-grid generators and you can take the nuclear element out (and it would last longer but still be radioactive).
skgoa 8y ago
Both, really. Having more ceels gives you reserves obviously, but since the per cell load is reduced, each cell degrades slower.

That's interesting! Is it like an SSD where you plan for a certain amount of cells to fail or does it reduce load on the cells so they last long individually?

by foota 8y ago

That's interesting! Is it like an SSD where you plan for a certain amount of cells to fail or does it reduce load on the cells so they last long individually?

TheSpiceIsLife 8y ago

Generally speaking, lead acid batteries, and lithium batteries, last longer if they are never (rarely?) deeply discharged. If you need 100Ah from your batteries you’re better off provisioning as many times more up to some cost-benefit analysis.

derefr 8y ago

The latter. Whenever your battery gets super discharged, picture this happening inside it: https://www.youtube.com/watch?v=r-YbQN_twpw https://www.youtube.com/watch?v=r-YbQN_twpw Notice how running the current the other way will break up the formed crystals, but it won't actually re-dissolve them. That's what it means for a battery to wear out. (Battery electrolytes are chosen specifically to be resistant to this, so the effect is mainly visible as a thin "rust" of deposited crystals on the battery's anode, rather than the full-scale crystals seen here.) A similar but distinct process occurs when overcharging a battery, where, instead of splitting the electrolyte molecules apart, you're reacting and bonding them together to form new molecules or molecular complexes, with the reaction usually being one-way rather than an equilibrium reaction. (For lithium batteries this process is exothermic and catalyzed by the presence of the product—thus lithium battery explosions.) It's much better for the life of the battery if it basically just hovers around 40-60% charge for its whole useful life, since then you're just generating tiny seed crystals (on discharge) and then re-dissolving them (on charge), where those crystals are small enough that they can be fully re-absorbed. And this is true even in battery-cell technologies that require a "deep charge cycle" to erase their "battery memory." Battery memory is basically the electrolyte causing enough crystalline rust specifically on the anode to increase its resistance. A deep discharge can capture and erase this rust—but it still shortens the battery's lifespan, because you're still producing non-reabsorbable large crystals within the electrolyte.

foota 8y ago
Thank a lot for sharing! That was interesting. So the desire to hover around 50% requires 2X larger batteries as would otherwise be necessary?
- mikestew 8y ago
  Yup. Though I've heard (as it concerns deep-cycle batteries) not to let them drop below 50% charge rather than keep them in a certain range. So, for example, when I purchased a new house battery for our RV, I got a 200ah battery knowing I can only make use of 100ah before recharging. Or put another way, figure out how much you'll use between charges (or in the case of solar, how much to tide you over until the sun comes up), and double it.

zaarn 8y ago

Not quite. Planning for battery failures is hard because they have different failure modes. For example, the battery could fail to dead short, which is the functional equivalent of an cell in the SSD dying and then setting the SSD on fire. Or the cell might just actually catch fire during normal operation if you overload it. To my knowledge, the best and safest battery system is to have each battery individually monitored by a charge controller with the capability to fully disconnect the battery at will. The downside is that this will be expensive so people usually settle for just eating the rare chance of a dead short battery. Additionally, batteries that you just leave around doing nothing will probably die at some point too. You don't want it sitting around you want to use it for efficiency or else you swap your battery and find out it was dead too. To reduce burst loads on batteries you could use supercaps if you find ones that can handle the voltage and current (that will be very expensive).

derefr 8y ago
> Additionally, batteries that you just leave around doing nothing will probably die at some point too. This wouldn't happen if batteries were designed to act like nuclear reactors, where the "fuel" (the fuel rods; the electrolyte) can be completely removed from the substrate that makes it react (the neutron medium; the anode+cathode.) But a battery that can withdraw its anode+cathode from the solution would be damned expensive. It'd make more sense as an architecture if you had just a few, super-large battery cells, e.g. giant vats of lead-acid. It might be possible to design regular battery cells such that they wouldn't start degrading until they were first exposed to a voltage load, though. (I think the "50 year" Duracell NiCd batteries have this property—they probably have an antifuse oxide layer between the anode/cathode and the electrolyte, that gets broken down when you put load on the circuit.)
- zaarn 8y ago
  That reminds me of a recent NASA project where they kinda built small nuclear fission batteries with less than a couple tons of weight (IIRC down to 100kg) These would be kinda neat as off-grid generators and you can take the nuclear element out (and it would last longer but still be radioactive).

skgoa 8y ago

Both, really. Having more ceels gives you reserves obviously, but since the per cell load is reduced, each cell degrades slower.