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Hi there, I’m in the process of building a power tank for my field Observatory, and I need some advice regarding appropriate fuse protection.  Power comes from two 10ah Deben Tracer LiPo batteries.  The batteries will be connected in parallel with one in-line fuse attached to each power outlet.  The circuits will then be split to supply WiFi and Mount on one side and dew control on the other.  Circuitry will be run through the following marine rated fuse box:

 

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Circuits then run from the fuse box to 12v cigar type power units. Here’s the sticking point, the blade fuses that might be relevant for this application are rated at 1, 2, 3, 4, 5, 7.5 and 10 amps and I’m having difficulty deciding which I need.

Circuit A  - Contains the Kendrick dew controller which is rated at 7A Max by Kendrick themselves.

Circuit B - USB 2.1A/1A

Circuit C - Contains the Sphinx Mount, rated at 1.7A max (derived from the max rating on the power unit supplied by Vixen).

Circuit D - The WiFi supplied by a TPLink   TL-WR702   router, which draws approx 0.26A maximum.  Adding these two elements together comes to a total of 1.96A for circuit B.

On the face of it no combination of fuses seems to cover the relevant loads.  It will be pretty obvious to anyone reading this that electrical DIY is not within my comfort zone ( as a software engineer I used to get accused of being; along with my fellow softies, ‘a fake engineer’ by my EEE qualified engineer boss lol.  Any advice on this conundrum would be most welcome ?

Kind Regards

Paul J.

Edited by DeepSkyMan
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I used to fly RC electric helicopters in competition and I have seen some serious fires when connecting lipo batteries in parallel. If one of the batteries is discharged and is connected to a charged lipo in parallel there will be a large current flow between batteries. I see that the batteries in question have some protection circuits built in so there might be a limited risk but take care. Have a read of this article for more info

https://oscarliang.com/parallel-charging-multiple-lipo/

 

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2 minutes ago, Tomatobro said:

I used to fly RC electric helicopters in competition and I have seen some serious fires when connecting lipo batteries in parallel. If one of the batteries is discharged and is connected to a charged lipo in parallel there will be a large current flow between batteries. I see that the batteries in question have some protection circuits built in so there might be a limited risk but take care. Have a read of this article for more info

https://oscarliang.com/parallel-charging-multiple-lipo/

 

Thanks Tomatobro, much appreciated ?

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Hi Paul,

I'm not a fan of the 12v cigar type connectors. I use the following:-

https://www.ebay.co.uk/itm/Cobra-XLR-Connector-Male-3-Pin/152431488063?epid=1139269400&hash=item237d9fec3f:g:~OQAAOSwEaBaBG9A

https://www.ebay.co.uk/itm/XLR-3-Pin-Female-Male-Socket-Audio-Connector-For-Microphone-Mic-Chassis-Panel/172816557599?var=471599051660&hash=item283cab661f:m:mPaGJqJAY1_ylpZa-KZGzYw

Regards fusing your various outlets.

Circuit A . I would check the maximum draw current with a multimeter and use the appropriate fuse just below the maximum reading. Alternatively, use a 5 amp fuse. I can't see any issues unless there's a fault.

Circuit B, C & D 2 amp 

Steve

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58 minutes ago, sloz1664 said:

Hi Paul,

I'm not a fan of the 12v cigar type connectors. I use the following:-

https://www.ebay.co.uk/itm/Cobra-XLR-Connector-Male-3-Pin/152431488063?epid=1139269400&hash=item237d9fec3f:g:~OQAAOSwEaBaBG9A

https://www.ebay.co.uk/itm/XLR-3-Pin-Female-Male-Socket-Audio-Connector-For-Microphone-Mic-Chassis-Panel/172816557599?var=471599051660&hash=item283cab661f:m:mPaGJqJAY1_ylpZa-KZGzYw

Regards fusing your various outlets.

Circuit A . I would check the maximum draw current with a multimeter and use the appropriate fuse just below the maximum reading. Alternatively, use a 5 amp fuse. I can't see any issues unless there's a fault.

Circuit B, C & D 2 amp 

Steve

Thanks Steve.  My original decision was 2A for B,C and D as well, but the 0.3 difference between the 2A FUSE and the Vixen power rating of 1.7 left me with concern.

Appreciate your comment on the cigar type connectors, I’m not a big fan of them either.  I’m viewing this implementation as v1.0, just using the kit supplied by the OEMs.  If v1.0 works sweetly, I’ll likely look at replacing the cigar interfaces in a later version?

Edited by DeepSkyMan
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Wiring two equal lithium batteries in parallel should be fine, but you MUST make sure they are both equally charged first! I’d fully charge them individually and let them rest over night so the voltages settle before connecting them. After that you keep them connected and charge them as a single unit. 

For the fuses you simply figure out the max current draw for each circuit (as you’ve done) and use a fuse that offers a bit of headroom, eg 10A for your dew heater, 3A for your 1.96A circuit etc. The role of the fuse is not to protect the equipment, but to break the current if the equipment malfunctions / short circuits and prevent a catastrophic failure such as a fire. 

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4 hours ago, martinl said:

Wiring two equal lithium batteries in parallel should be fine, but you MUST make sure they are both equally charged first! I’d fully charge them individually and let them rest over night so the voltages settle before connecting them. After that you keep them connected and charge them as a single unit. 

For the fuses you simply figure out the max current draw for each circuit (as you’ve done) and use a fuse that offers a bit of headroom, eg 10A for your dew heater, 3A for your 1.96A circuit etc. The role of the fuse is not to protect the equipment, but to break the current if the equipment malfunctions / short circuits and prevent a catastrophic failure such as a fire. 

Totally agree with martinl

About wiring batteries in parallel, you can add a diode to the positive output of each battery to avoid the incoming current

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1 hour ago, VIctor_M said:

About wiring batteries in parallel, you can add a diode to the positive output of each battery to avoid the incoming current

That’s an interesting idea. I’m not sure it would work well with a 3 cell LiPo like the Tracer though since the diode would cause a ~0.6-0.7V voltage drop and 3 cell LiPos only reach 12.6V fully charged.

For 4 cell batteries that have voltage to spare above 12V it might work. My 4 cell LiFePo4 sits at around 13V for much of its charge, for example. The downside would be that you’d need separate circuits for charging. 

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Guys, thank you everyone for your contributions to this thread.  I think the basics of original circuitry should meet the demands and associated risk for this exercise, crucial though as pointed out will be the in-line fuse protection between the batteries and the fuse box.  The point about the equality of charge across both batteries is not something I would have thought about, but again the inline fuses should provide sufficient protection on this score as well.  Lots of food for thought here, for further down the line perhaps for version 2.0?

Kind Regards

Paul.

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