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I've got a quite delicate setup with a PC and some other minor equipment such as a microcontroller connected to a large 100Ah sealed (AGM) 12v lead acid battery.

I need to be able to run my equipment non-stop for months and only charge the battery at night time. I would love to get some advice on how to run this setup as smoothly and safe as possible.

  • Is the best way to just leave my stuff plugged into the battery and just hook up a charger to the battery as well?

  • Should I use two batteries and just swap them, plugging in a fresh one in parallell before disconnecting the old one? And that way I can just charge them one at the time.

The first solution feels easier but I'm not sure how my sensitive equipment handles the plugging in and out of a charger. Or the swapping of batteries for that matter.

Are there some good practices for creating a setup like this? What precautions should I take, do I need a diod to prevent surges while connecting a charger? Do I need any voltage regulators to prevent bad things happening?

Grateful for all advice on this. Thanks.

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There are many power supplies available for this purpose. As 0x6d64 says, these have a constant voltage output of around 2.3V per cell, ie 13.6V or 13.8V for a 12V battery. The power supply will need a current limit so that the battery will not be damaged if it is charged when flat. This would typically be about 0.1 of the capacity - so no more than 10A for a 100AH battery - probably 5A to be safe.

You can safely leave everything connected 24/7 with this arrangement and switch the mains on and off as necessary but some deep-discharge protection is advisable. This would cut the power output if the battery voltage drops below about 10V to prevent battery damage. Of course your connected equipment needs to be capable of operating at 13.8V for this to work. If 14V is the absolute maximum then this solution is borderline.

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  • \$\begingroup\$ Lead acid is not my specialty but I thought they did not suffer permadeath from deep discharge like other battery technologies can. \$\endgroup\$ – Kortuk Aug 18 '11 at 13:26
  • \$\begingroup\$ @Kortuk - I'm no expert in battery chemistry either but according to my Yuasa battery manual "severe sulphation" can occur if the battery is discharged below the recommended cut-off voltage which increases the internal resistance and may prevent the battery from charging. This is made worse if the battery is left discharged for extended periods. \$\endgroup\$ – MikeJ-UK Aug 18 '11 at 14:24
  • \$\begingroup\$ you should add that to your post and give a link. Explain why the cutoff is needed and with a link you will look like you normally do peer-reviewed articles. Wait, this is peer reviewed! \$\endgroup\$ – Kortuk Aug 18 '11 at 14:26
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A good precaution would definitely be using fuses and place them close to the power source (e.g. battery or charger).

If you need some voltage regulation depends on the kind of equipment you'd like to supply: If they have an absolute maximum input voltage of 12.0V you will need to regulate that voltage (low dropout regulator? depends on the current, I think).

For charging: I personally charge with a constant Voltage, because it is simple. This might not work for you, as it is quite slow. What state of charge do you expect every evening?

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  • \$\begingroup\$ My equipment takes a variable input of around 10v to 14v. Fuses are sensible idea. I'm gonna be pulling around 5Amps during up to 12 or 14 hours which should put the charge down as low as 20%. \$\endgroup\$ – Mattias Aug 18 '11 at 6:44
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As AGMs are not sensitive to deep discharges they do not need special precautions.

I do not know what you mean by sensitive. Assuming you mean ripple-sensitive. I would use a charger that is especially made for that purpose, eg. a charger that would charge the battery slow and not at the higher voltage used for "normal" charging.

Connecting another battery at 13 V to a battery at say 12.3 V could definitely introduce some electromagnetic disturbances.

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