Quote:
Originally Posted by Swagman105
I have plenty of 18v (Ozito) and 20v (Aldi) power tool batteries for my collection of power tools.
Wondering if anyone knows of a way to provide a 12v output from these batteries to power my telescope mount?
Also can anyone foresee any problems if an adaptor could be made or sourced?
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Yes of course it can be done, but depending on how it's done it can be somewhat wasteful of the energy stored in the battery.
So let's say you want 12 volts DC at several amps. You may actually want more than 12V for optimum mount performance, but that's another story / possible risk.
The 12V supply to the mount should be regulated this can be done in a few ways, but 2 that come immediately to mind are
Linear Voltage Regulator
This a typically a 3pin electronic device (some 4) which requires a few volts (1.5-3V) higher of a input voltage than the required output voltage and can produce a very precise output voltage, either at a specific voltage depending on the regulator chosen or can be an adjustable, often 4 pin type. You can have a higher differential than the 1.5 to 3Volts, but it gets wasted as heat and too low a voltage differential causes loss of precision regulation (drop-out). Since these types of regulators regulate by shedding the voltage excess above the control voltage essentially as heat, they would be very wasteful energy wise in a remote power/battery application and DC-DC converters are probably preferable unless ultra low noise and precise regulation are also an important consideration (unlikley in the suggested application). They are ; however, excellent in low noise precision applications like audio, etc...Typical types are LM7905, 7912, 7915, etc.. with the last 2 digits (LM79
XX) referring to the output voltage. These usually only have output currents of 1 to a few amps, but can be supplemented by external pass transistors for very high output 10-15Amps +...
DC-DC Converter
These DC to DC /buck converters as they are known, are far more efficient, especially in the presence of a large voltage differential between input and output when compared with linear types. They are however
possibly more noisy and not as precise in regulation as the Linear types above ,but are available as ready built modules on the likes of eBay and possibly via electronics stores like Jaycar.
A typical 18V power tool battery is any where from 1.5 to 5 Ah with 4 and 5Ah types typical at the larger end, these equate to 72 to 90Wh respectively of energy storage. Using such an 18V battery with a linear voltage regulator one would probably only access 40-50% of that available energy as it would only properly regulate when the 18V battery was in the 20 Volt to 13/15 Volt range: i.e: fully charged at 20 or so Volts down to the drop-out voltage of the regulator anywhere between say 13 to 15V for a 12Volt regulator like an LM7912, whereas a DC-DC buck type converter would use far more of the available enerrgy whilst still maintaining regulation. The extent to which it does this depends on the specs for the module.
Even when you have all your ducks in a row with the right/best choice of DC-DC converter, how much can you expect from a typical 18V 4-5Ah battery? The mount draws something less than 1 amp tracking and ~<4 amps slewing as I recall from a video I once posted. I wrote about this once - I need to find that previous reference video to add further later.
This is not the video I was looking for, but has some answers regarding current draw (although no mount payload is specified), it shows 0.37A in rest state (not sure if this is when tracking or stopped) and just over 2A when slewing at highest speed (rate 9). On another occasion I did find a video on youtube (now no longer there unfortunately) which showed 3.48 A being drawn during full speed slewing on an unloaded AZ EQ6 mount.
If the 0.37A mentioned is the current draw whilst tracking or even perhaps a little higher, then your 18Volt 4 to 5Ah battery with DC-DC Buck converter with the odd bit of slewing should keep you going for a reasonable night session of a few hours.
Food for thought ....
It might be possible with some experimentation and with EQUALIZED batteries (equal voltage & equal condition/health) and some risk to parallel a few batteries for increased energy storage, but I'm not 100% sure how some of the Li-ion battery packs' control systems would like that. It might depend and be risky. In some situations a good battery might be degraded by a lower health battery it's paralleled with. Enter at your own risk on that one.
Best
JA