I have a project (remote gate opener...) I have made good progress but its the first time I have built a charging circuit so I have some questions I need help with.

The battery I am using is a sealed lead acid battery NP1270M datasheet here

The gate will be near a power source and i will supply 18VDC @1.5A as an input to the circuit. The project has a Micro controller so I plan to implement automatic overcharge/undercharge safety. As well as switching I will limit the voltage and current (fixed amount). The battery will never be less than 80% so I was just going to trickle charge at a fixed rate until it reaches a voltage or the voltage no longer increases.


  1. What voltage should I use for "Fully charged" can't seem to find that anywhere i.e. if reached turn off charging.
  2. What voltage should I use for under-voltage protection (<12v?)
  3. What voltage and current should I trickle charge at note: 30% in 8 hours will be quick enough.

I don't understand this from the datasheet:

enter image description here

Thank you in advance for your help.

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    \$\begingroup\$ batteryuniversity.com Lots on lead acid \$\endgroup\$ – Russell McMahon Apr 6 at 23:46
  • \$\begingroup\$ Yes I started there when trying to get answers to full charge voltage was frustrated I couldn't find what I needed. thanks. \$\endgroup\$ – AndrewT Apr 6 at 23:50
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    \$\begingroup\$ look at the charging characteristic chart \$\endgroup\$ – jsotola Apr 6 at 23:50
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    \$\begingroup\$ FWIW that's a much better data sheet than many. If they produced it themselves and did not just copy it it suggests a better than average quality battery. IF. \$\endgroup\$ – Russell McMahon Apr 7 at 0:08

That's a nice datasheet. Trying to work out what the curves mean would be educational - but it's not essential for your purpose.

In your application you are essentially "floating" the battery.
The other typical use is deep discharge.
Your application lies somewhere between but closer to floating.

13.5 to 13.8V maximum voltage when charging (float charge) is good enough for your purpose.
Charge at not more than 1.75A and ideally at around 0.7A until this voltage is reached.

Occasional increase to 13.8-14.1 (Equalise or boost) will help maintain capacity, but is not essential.

Do NOT float at 14V plus - that is only for recharge after deep discharge.

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  • \$\begingroup\$ Hi thank you, sorry struggling to follow I asked three questions above are you saying the final charged voltage should be 13.5-13.8 or are you talking about the chargers output voltage? \$\endgroup\$ – AndrewT Apr 7 at 0:34
  • \$\begingroup\$ On second read I think you are answering #3 charging voltage. Thank you \$\endgroup\$ – AndrewT Apr 7 at 0:55
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    \$\begingroup\$ See modified answer. Charge at 1.75A max but preferably about 0.7A until V is in the 13.5 to 13.8V range. \$\endgroup\$ – Russell McMahon Apr 7 at 1:31

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