In principle is it possible to recharge AA/AAA/9v batteries in the device they are currently plugged in?

There are numerous smart chargers for AA/AAA and 9V batteries. The process requires taking all batteries out charging then putting back in. When you are potentially dealing with multiple such devices the experience can only be bettered by carefully modding any device to recharge batteries and skip on the in out routine.

Specifics to test this in my case is a 3V - 2xAA device(vacuum dust collector) and an "Intelligent" battery charger with 12V input and 4x AA/AAA charging slots.

So I am happy with modifications of any sort with the ultimate outcome of : 1)Allowing in device recharge 2)Allowing greater distance between where the device is resting and where the charger unit is. Think of USB charger + cable as a guide 3) Make the principle generic so it can be used on any device and any charger

Extras : If possible use a high capacity battery bank(26800mAh for examples sake) to power as many battery slots on the AA/AAA charger. Battery bank used in this example is a 3x USB with output DC 5V/5.5A(max) total and 2.4A individual USB total.

Please check photos for more clarity

The challenges I am aware of : 1) In the device AA/AAA batteries are connected in series with a simple metal piece spanning over both battery terminals. The 4x AA/AAA charger connects to each batteries +/- poles individually. So this might be a problem as the charger needs to adjust output to each individual battery's condition ...

How do you temporarily ( mechanically or electronically ) gain individual access to each battery given they are all connected in parallel inside the device? Modding/Reworking the insides of the device is fine... should be generic solution as most devices use the same Spring to metal plate hardware to raise input voltage from multiple AA/AAA batteries.

2) USB Cable length effect on the AA/AAA charger output. The charger expects batteries to be in the slot and not 1-2m away. Even if the charger might be capable of delivering whatever voltage is needed to offset the length etc. Being an "intelligent" charger means it may interpret the new circumstances wrong ... project setupmore devices to hack

  • \$\begingroup\$ Beware shooting yourself in the foot! ....I couldn't resist! :-) Keep on hacking! \$\endgroup\$ Aug 6 '16 at 20:46

While there are complex ways of achieving this, a simple method that is good enough for many situations is possible. Chargers that charge 4 x AA or AAA cells usually do so either with

  • All 4 cells with eg negative grounded and separate positive feeds or with

  • 2 x 2 cells in series.

Below I use "toy" to refer to the target device to be charged. Could be model car, Pokemon, toothbrush, fan etc.

Appliances may have 1 or 2 or 3 or 4 (sometimes more) cells, usually in simple series. Some device use a centre tap for 1+1 or 2+2 etc cells. Examples may be model toys with motors that change direction and a split +/- supply is used rather than using eg H-bridge drivers.

In the case of Nt batteries in series in the "toy" (1 or more) and Nc batteries in series in the charger it is not usually feasible to convert the toy to match the charger connection pattern if there is not initially a direct "mapping". This would involve placing isolators between the cells at appropriate points wit switches across them and a wiring 'loom' to the charger.

The simplest and not vastly expensive option is to provide X isolated chargers whose outputs float relative to all the other chargers outputs. These would usually be capable of charging 1 or 2 cells in series.
Each toy then has a wiring loom that maps either individual cells or pairs of cells onto their own isolated charger.

The key is that all outputs are isolated from all other outputs unless connected in some desired manner.


If/when this gets hard and/or mind boggling

  1. Realise that somehow Sir Isaac Newton could have visualised this without effort.
    Marvel. Then ...
  2. Draw a picture.

Provide 4 isolated chargers able to charge 1 or 2 cells in series. Each charger has connects Cxg = Cx ground, Cx1 = charger x 1 cell +ve and Cx2 = charger x 2 cells +ve.

The following is "doing it the hard way" for example's sake.
In this case IF the chargers can charge two cells we could have used just 2 chargers for a single series string of 4 cells. I've used 4 chargers as if each can only charge 1 cell.

A toy with 4 series cells B1 B2 B3 B4 with B1t = B1 top (+ve) and B1b = B1 negative is used with cells arranged.
+ve - B4t~B4b B3t~B3b - B2t~B2b - B1t~B1b - -ve.

Call the connection points T+ T43 T32 T21 T- You can work out what that means :-). Wires are brought out from the 5 termination points to a universal connection socket. Each toy has a socket wired to suit the charger banks standard patterm.
In this case the pattern is

 |     Termination point             
 |     |          

C41 - T+
C4g - T43

C31 - T43
C3g - T32

C21 - T32 C2g - T21

C11 - T21
C1g - T1g

Each charger "sees" a single cell.

You COULD have used

C22 - T+
C2g - T32

C12 - T32
C1g - T1g

The wiring to suit each toy is in its socket. The 4 (or more or less) are chargers are wired in a standard manner with each output isolated unless joined by the socket in use.

Example only:

4 x floating chargers at left have mutually isolated outputs. Connection to chargers is by a consistently wired "plug" - in this example, 8 pin.

At right are two examples of loads.
Load 1 middle has 4 series cells and 4 x 1 cell chargers. Wiring from cells to "socket" connect the 4 cells so that they 1:1 "map" onto the chargers.
Load 2 far right uses 2-cell chargers. Cells are mapped by wiring so that top two cells connect o one charger and the other two cells connect to the other.


simulate this circuit – Schematic created using CircuitLab

This principle can be extended to 9v or 12v or ... batteries as desired.

  • \$\begingroup\$ I like your choice of 'toy' as a reference. 'Pro' toy sounds about right to me :). The charger used as example says "Professional series intelligent AAA/AA battery charger, each battery is controlled & monitored independently". Its 4 batteries in series as the charger sees it , one charger per Pro-toy cell. What got me thinking I can get away with one charger per 4 toy cells(if the toy has 4 cells same as the charger) is the wording "individually controlled and monitored". It can monitor individually a battery when in series but how would controlling it individually not affect others in series \$\endgroup\$
    – Nick
    Aug 6 '16 at 20:38
  • \$\begingroup\$ I am yet to figure out your full post here so Newtons reference comes handy too :) \$\endgroup\$
    – Nick
    Aug 6 '16 at 20:39
  • \$\begingroup\$ Still not fully got my head around this but I think you touched on the option I was "toying with". Using a "reverse transistor" switch. That is when voltage is not applied to a 3 pin terminal the current flows as the toy battery compartment was originally designed and when Voltage is applied to the "reverse transistor" switch all 4 batteries become isolated inside the toy. Once isolated inside the toy I would simply connect 4x2 wire cable back to the charger and make it seamless as far as what the charger can "see". Perhaps there is such a "reverse transistor" switch? Is this a feasible way? \$\endgroup\$
    – Nick
    Aug 6 '16 at 20:54
  • \$\begingroup\$ @Nick You could use isolating switches in the toy BUT that's not what I intended. The key point in my system is that the N charger outputs all float wr each other. So eg if you had a LiIon charger that charge single LiIon cells you could map 2 x such chargers onto two hard series connected LiIon cells. This is a common arrangement in higher spec cameras (DSLR etc)(but they don't give you the cell mid point. OR you could us 1 x 2 cell LiIon charger. All the "mapping is done by wiring to the battery side sockets. See added diagram. \$\endgroup\$
    – Russell McMahon
    Aug 7 '16 at 13:52
  • 1
    \$\begingroup\$ @Nick The charger you describe is not necessarily flexible enough. You charge each independently with suitable monitoring and the ability to inject current INTO OR take current out of each internal node. So eg if top battery needs 500 mA and next down needs 300 mA you feed 500 mA in the top and take out (500-300) = 200 mA at the jn with the next cell. BUT they say "each battery is controlled & monitored independently" -> This MAY mean they float independently but also may mean the electronics is flexible as above. If eg cells are 2+2 banks isolated charger may need true inter-output isolation \$\endgroup\$
    – Russell McMahon
    Aug 7 '16 at 13:58

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