Nimh batt charging

[joeh]

hi all
i bought a batt charger with rechargeable AAs and plug into 240v wall socket with no problems. i then brought it to boat n plug into 100W inverter but it doesn't charge. any idea as to whats wrong?

 

[Evadne]

First thought is that it needs more than 100W. But although this is just under half an amp at 240V it is loads more than you will ever need to charge a 1.2V cell. It is possible that the startup current for the charger power supply at 240V is more than 0.4A however, e.g. a big input capacitor on the AC side of a switch mode power supply, If possible try a bigger inverter or measure the current on the AC line (not easy without a clamp meter).

Other thoughts are less likely causes: dirty waveform from the inverter, spikes on your inverter blowing up the charger.

 

[Joe_Cole]

I don't know, but it seems an inefficient way of doing it to convert 12volts DC, to 240volts AC, and then to convert it back down to 1.2volts DC. If you are going to do this frequently why not get a charger that you just plug into the boat battery socket?

 

[Guest]

12v charger ...

Get yourself to a Radio Control Model shop - proper job - not the silly Airfix plastic models shop !! or on-line. You can ask for a Field Charger .... which is a 12v powered AA charger that will charge 4.8 or 9.6V AA packs......... used in RC Radio sets. Then a trip to maplins or even same shop will supply battery box that you can put 4 aa's in .... you wire that to the 4.8v output. Designed to charge a std 500-600mAh set at 10 hrs .... but in fact you could leave much longer without damage.......

I have one on the boat - the 9.6v charges the VHF HH, the 4.8v charges sets of recharge aa's for various .... via the battery box.

QED /forums/images/graemlins/blush.gif

 

[William_H]

Re: 12v charger ...

Following on from Nigel's suggestion. If you get a battery holder from Maplin then you can connect to the 12 volt supply via a resistor.
The value of the resistor is chosen from the formular resistance equals voltage divided by current.
The current can be be chosen by dividing the Amp hour rating of the battery by the time you wish to charge the batteries and add about 20%.
The time you choose to charge your batteries is compromise. 3 hours for a fast charge while in excess of 10 hours means that the batteries can be left on charge long term without too much damage. ( At the 3 hr rate you will ruin the batteries after say 4 hrs. So you choose.
So 1300mAH batteries at 10 hr rate should be charged at 130 ma say 150 ma.
The voltage for the formular is the ships voltage 12 volts or 14 volts depending on charging conditions. minus the battery (being charged) voltage. So 14 volts minus 4.8 volts for 4 cells equals 9.2 volts divided by 150 ma (9200/250) equals 36.8 ohms. Resistors are sold in prefered values so a 39 ohm resistor is what you want ( or 47 ohms). The power wasted in the resistor is given from current (250 ma mutiplied by voltage 9.2 volts is 2.3 watts. So you need to buy a resistor rated at 5 watts ( the more the better) and allow that it will get warm. ( don't be confused because you specify the resistance, the power rating only signifies how much it can take before cooking not how much power it will suck from the system as in a light bulb. So all very simple at total cost about a squid you just need to be carefull about charging times. If you intend leaving on charge permanently I would go for a 20 hr rate( ie double the resistance). If you have a multimeter you can use it on current scale after breaking in to the wiring.
Another good trick is to buy a cheap LED and connect it across the resistor via a 220 ohm ohm resistor low wattage. This will glow when the batteries are charging. NB the positive of the LED must go toward the 12 volt suply.
regards olewill

 

[mikejames]

Re: 12v charger ...

I am using a Vanson V1000 charger from Maplins which fast charges 2 or 4 AA or AAA cells. It stops charging when the cells warm up (and the voltage dips) so it can cope with anything from old NiCd (about 1 hour) to latest NiMH (4 hours+). It has a separate 12 volt mains adaptor and came with a cigar lighter lead as well, so it plugs straight into the boat or car. It came in a blister pack with 4 AA cells for about £20 a few years back.

The problem with cheap inverters is they produce a rough noisy squared off 'AC' wave . The roughness of the AC may be making the charger think the battery voltage is varying , and reaching end-of-charge , if it is a smart charger.

 

[Guest]

Re: 12v charger ...

Sounds about right !!

The resistor idea is a good one from another post above .... but I'm terrible ... I know the calculations having done DC and AC theory at College many moons ago - but still don't trust my own calcs !!
The Field Charger that I proposed is a safe and concenient way to do it ..... will charge 8 cell VHF on one connection - (just check polarity on VHF - as many swap the pin !!) and 4 cell on the other. Its safe to leave all day or night, has LED's indicating charging and is in a nice small about 1.5 inch square box..... forget what it cost but compared to many 'marine' items - peanuts !! .... gives peace of mind !!!

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