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running current of a Synchronome system is about 1 or 2 mA. The ‘peak’ capacity needed maybe 500 mA. These are well within the capacity of modern AA rechargeable cells, which can be float charged from an old mobile phone or similar adaptor. I fuse for 0.5A in a circuit with a minimum wire size capable of carrying 3 A. The battery itself (it powers more than one system) is fused for 1.0 A.
I am assuming your 90 Ah battery is lead acid - so it will have a very low internal resistance - and as you rightly say can deliver massive currents under fault conditions.
I use ‘C’ size rechargeable in my main system - from memory NiCd, but may be Nickel Metal Hydride (they are not all that easily accessible). They have been operating fine for over 10 years. I have also used AA size Nickel Metal Hydride (for a different system) which proved very adequate and both systems have covered power cuts of many hours without loss of function.
My clock system has two 90 Ah lead acid batteries. The very first thing I did was to put inline fuses on the battery connectors. I also do the same for the isolation transformer if testing an old piece of electrics for the first time. I have blown a few fuses while fiddling about, even being careful. Better that than to test the short circuit capacity of a 90 Ah battery which I assume would be in the order of hundreds or likely over 1000 Amps on a dead short.
Simon GPO clocks
On 23 Sep 2019, at 01:38am, Tim <elliott2d@...> wrote:
An isolation transformer is a transformer that has the primary winding and the secondary winding electrically isolated. The ration of the primary coil to the secondary coil determines the output based on the input. A ratio of 1:1 means that if you feed the transformer with 110 VAC the output will also be 110VAC but electrically isolated from the power grid on the primary side. Isolation transformers are typically rated for a certain load or current and a voltage. Common isolation transformers have ratios of 1:1 (most common), 1:2 and 1:10 but like most transformers can be obtained is pretty much any ratio you want or need. The isolation level is usually rated in voltage at which the transformer can stand off. Many of the transformers have an isolation value of 1KV, meaning the input supply can be referenced to earth ground but the output reference can be floating at up to 1000 v above earth ground. Most Isolation transformers are used to help reduce noise on the instrument on the down stream side such as an oscilloscope and are therefore capable of only a few amp of current output. One of the main uses we have for isolation transformers is to 'float' test instruments at the voltage an experiment is operating at, ion sources are one of the primary uses for me. The current and voltage ratings are what the transformer can be used at with out damaging the unit. Isolation transformers only break the electrical link from the mains (input side) to the output removing a possible fault current path when testing or operating a unit. They do not protect the operator from electrical shock so you need to be aware of any current paths in a fault condition. Using an Isolation transformer to test a device prevents (or reduces) the chance of a catastrophic should the device under test have a electrical fault that would pass mains current/voltage to the chassis and/or ground
Simon GPO Clockshttp://www.lightstraw.co.uk/gpo/clocksystems/index.html