choosing battery type for 50Wh, small project

[Laibsch]

How does a lead-based battery compare with other types for storing about 50 Wh of energy?  NiMH or Lithium-based designs such as the CR123 were mentioned earlier.  The amount in question would be fairly steadily withdrawn over the course of about 5 to 10 hours (powering a netbook).  Replenishment would happen immediately or after a few hours at most (diesel-driven generator kicking in at 6PM).

[ghurd]

50WH does not seem like nearly enough for powering a netbook 5 to 10 hours.

Lead-based is cheaper, and easier to charge than lithium based cells.
Maybe a 86AH SLA (7.2AH-class) would be worth experimenting with?  Decent used ones are not too hard to find locally.
G-

[Laibsch]

Ghurd, thank you for your reply.

The battery of my netbook is 4400 mAh at 11.1V which is roughly 45Wh and it keeps the netbook running for around 11 hours under ideal conditions.  Furthermore, 50 Wh is the rough maximum to be withdrawn from the battery over the course of the day.  There will be a small (2W?) solar panel as well to provide some power (the place is always sunny) so the battery will not be the only source of power.  I hope this makes more sense now as to why I gave a figure of 50 Wh.  In the end, if the battery is empty, it will be empty and I would just need to wait until 6PM for the generator to kick in.

My understanding is that lead-based batteries do not appreciate if the full capacity is taken out.  So, I wonder if a 7.2 Ah SLA wouldn't be seriously undersized.  Calculations in IRC were based on a DOD of 10%.  That would substantially increase cost and weight of a suitable lead-based solution.

[WooferHound]

If I'm calculating it right . . . 50wh at 12vdc is 4ah.
I think a standard cheap car battery would work fine.
they are somewhere around 35ah.

[ghurd]

The battery of my netbook is 4400 mAh at 11.1V which is roughly 45Wh and it keeps the netbook running for around 11 hours under ideal conditions.

My understanding is that lead-based batteries do not appreciate if the full capacity is taken out.  So, I wonder if a 7.2 Ah SLA wouldn't be seriously undersized.  Calculations in IRC were based on a DOD of 10%.  That would substantially increase cost and weight of a suitable lead-based solution.

If 4.4AH runs it 11 hours, then 400ma?
On a good 7.2AH, thats about 18 hours instead of 5 to 10 hours.
Myself, I'd just try to keep it above 12.25V, bit over 50%DOD or so.  That would be at maybe ~8 hours?
2 decent used ones should do it?
Like I said, might be worth experimenting with.

weight:  In my mind, 5 to 10 hours meant it was a stationary netbook.

(the place is always sunny) may mean more solar than that would be a good idea?
G-

[MadScientist267]

Something to keep in mind with SLA - They do not like being discharged at anything greater than C/20, and charging them much faster than that almost always has made me wish I hadn't.

For a 7.2Ah, this means ~360mA. In my experience, and as G mentioned, SLA is happiest when cycled to 50% on a typical discharge. You don't want to yank them down too far regularly, but they get "lazy" if they don't get a workout too.

I'm with Woof on this one... If you're not too worried about portability, just use a car battery. Even though they aren't meant for deep cycling, you're unlikely to take it far enough down to shorten the life of it all that much over time. Just don't use one that shares terminals with a starter, just in case.

If you decide to go SLA, sizing is the key; Its just one of those things.

Steve