Quote:
Originally Posted by Steffen
If you extract 7 ampere hours from a 7.2Ah SLA battery (like the Jaycar guy seems to suggest) the battery will be pretty much a write-off.
|
I'm not sure that's correct for SLA batteries.
The actual effective capacity of an SLA depends on the
average current drawn from it. E.g. for the Jaycar 7 Ah battery, they quote the following capacities:
- 20 hours @ 0.36 A = 7.2 Ah
- 10 hours @ 0.648 A = 6.48 Ah
- 5 hours @ 1.152 A = 5.76 Ah
- 1 hours @ 4.32 A = 4.32 Ah
http://www.jaycar.com.au/products_uploaded/SB2486.pdf
Most telescope mounts draw 1 amp (or significantly less) on average when tracking at sidereal rates (peaking at over an amp for full-speed slewing etc), so a 7 Ah SLA is probably good for 10 hours or so of constant use. (YMMV)
If you get in the habit of recharging every night or two, you should have no problems, and SLA batteries are very tolerant of shallow cycling:
"They do not suffer from the memory effect displayed by NiCad and NiMH batteries, and are therefore quite suitable for shallow cycling applications where they spend most of their time connected to a trickle or float charger. In fact unlike NiCad batteries, they prefer shallow cycling. This makes them much more suitable for emergency-standby applications such as UPSs and emergency lighting systems."
http://www.jaycar.com.au/images_uploaded/slabatts.pdf
Also:
"Although SLAs do prefer shallow cycling to deep cycling, they are nevertheless capable of supplying occasional heavy discharges without adverse effect."
...
"Generally speaking manufacturers recommend that SLA batteries are not stored in a discharged state, nor allowed to remain in such a state for very long."
[My emphasis]
http://www.jaycar.com.au/images_uploaded/slabatts.pdf
Nevertheless, SLA batteries are cheap enough that it is easy to buy an over-sized battery, just to be on the safe side.