[mythtv-users] Netbook as low-power combined back-end / front-end?

[Simon Hobson]

rsh1k wrote:

>Thanks for the detailed reply, Simon!   I hadn't even thought about
>the computer's load fooling the protection circuits of the charger...

Just to clarify, it's not the protection circuits, it's the control circuits.

The "smart" lead acid chargers use a multi-phase charging regime. 
Starting from a very flat battery, they will charge in constant 
current mode until the battery voltage reaches some specific limit - 
typically around 14V for a 12V battery. They then switch to constant 
voltage mode while the battery continues to charge - the current will 
reduce as the charge state increases.
At some point, the current will drop below the threshold where the 
charger determines that the battery is fully charged (the charger 
needs to be matched to the size of the battery). The 'charging' 
current is no going into electrolysing water into hydrogen and oxygen.
The charger will now switch to float mode (around 13.8V) which will 
keep the battery fully charged.
Note: some have more sophisticated regimes, but the above should give 
you the gist of what the problem is.

If you have a load running from the battery/charger combination, the 
current as sensed by the charger will never drop low enough for it to 
detect end of charge. The charger will therefore stay in bulk charge 
mode indefinitely and you will "boil" the battery dry.

I'm sure there will be charger/PSU devices around which sense the 
battery current separately to the load current - hence they would be 
able to properly charge a LA battery while alos supplying a load. I'm 
also fairly certain that they'll cost "somewhat more" than a basic 
charger !

-- 
Simon Hobson

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