Why doesn't my battery charge? Why doesn't the charger's light come on?

Question edited for clarity and typos. Please don't call a Dyson a 'Hoover' (registered name of Hoover Inc), James Dyson will be very upset. :>D

Yes you can buy batteries very easily, but you need ascertain that it is the battery, or the charger, or the control board? With a digital multimeter it is very easy to check battery and charger, but you will need Dyson service to diagnose a board issue.

https://www.google.com/search?q=Dyson+DC35+battery

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You didn't give make model or serial number, so the information is going to be somewhat generalized:
Most charging systems go through what is basically a P.O.S.T.-a power on self test. This checks that the charger is working correctly, that the power coming in is correct, and that the battery needs a charge AND is ready to accept a charge-IE NOT overheated, or frozen. If any of them fail, ESPECIALLY the temperatures, IT will shut down, This is to prevent explosions! (THINK: a certain brand of phones and most hover-boards!)
What you can do: UNPLUG the charger from the vac, and the wall. Wait 24 hours. Try again. If it works, it was likely an overheated battery. If not, possibly the charger. Hard to say if it's worth repair, but that would be the place(s) to start.

Here is a video showing a quick temporary fix and the full fix showing the change of all the electrolytic capacitors.

I had the same issue, then i just found the manual, in charging part, it says:"The charging indicator will be lit as long as the
appliance is connected to the plugged-in
charger."
Also it says:"While charging, the charger may get warm, this is
perfectly normal and safe. It can remain on
charge indefinitely without damage or danger. It
is not possible to "overcharge" the battery cells
with the charger provided"

This condtion is caused by a defective/worn out battery, replace it.

Try running the battery all the way down by letting the vacume run in a corner or another room and even when it stops leave it on for a hour to drain the battery. Then shut the switch off and try charging the battery. Make sure the charger is putting out enough volts by looking at the rated output of the charger and while it is plugged in check it with a volt meter. If it does not hold a charge and the charger is putting out the right voltage you need to purchase a new battery for the vacume

If the charger is left connected without the connection to the battery for several hours and there is no heat on the charger, then it is possible that the battery is using up more charging current .
This results in heating of the unit for the large current delivery.
Checck your battery if the back up has dropped down, if so it is confirmed to change the battery.
Hope this advice helps

the charger is not charging its not putting any thing out into the batterys the charger clock is working but your not getting any charge from it

There's a chance that the battery you bought is bad. There's a greater chance that the problem is with the charger. If you're using the dock to charge, try plugging the charger directly into the roomba just to rule that out. If that doesn't fix the problem, I would try borrowing or buying a new charger, and see if that works. If a new charger and new battery don't solve the problem, the problem must be with your roomba itself.

If both batteries are behaving the same way then this is probably a charger problem. When you insert the battery pack into the charger does the 'charging' light blink for 30 minutes (fast charge) then go steady? I noticed that one of my batteries was causing the light to come on steady immediately I inserted it into the charger. When I tried to use the battery pack I got the same problem everyone else is seeing, i.e. vacuum cleaner shuts off in less than a minute, so I conclude that when the charging light is on steady then the battery is not being charged.

If the charging light does not blink when you insert the battery (i.e. comes on steady or not at all), try removing the battery and re-inserting it into the charger.

Try a conditioning cycle on your battery packs. To do this:
* Insert the battery pack into the charger
* Before the charging light lights press the 'condition' button located just above the charging light.
* The 'conditioning' light should blink. This is a light which does not normally illuminate, and it is located below the condition button and above the charging light. The charger fan will run quieter in this mode.
* Leave the battery pack in the charger for 24 hours while this cycle is taking place.

I'm not a service technician so perhaps someone else might like to post other solutions or corrections to this one.