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Discussion Starter #1
I have a problem/question re: the DC/DC converter and accessory battery setup.

See config and load info below. Here is the problem -
  • Turn car on *with headlights*
  • Fuse between DC/DC converter and accessory battery blows
  • Accessory battery rapidly loses voltage - within 20min
  • Lights die, along with relay for motor controller
  • Car stops

Car runs fine without the headlights.

I assume that the additional load on the accessory battery pulls more current from the DC/DC converter, which blows the fuse. And the battery is damaged so it runs down quickly.

Per info on lead acid batteries and talking with ExpertPower tech support, max amperage charging lead acid batteries should not exceed 0.25C, or 5A for a 20Ah battery. The tech support guy said I probably damaged the battery, even if I had a 20A fuse between the DC/DC converter and the battery.

What is the solution, since most DC/DC converters are >30A, and I don't see a lot of huge Ah 12V accessory batteries in the EV forum?

Thanks!


Config -

Load -
  • 1.25A - with ignition on
  • 7-9A - ignition and headlights
  • 10+A - driving the car with headlights, vacuum pump, PS pump, etc.
 

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I have a problem/question re: the DC/DC converter and accessory battery setup.

See config and load info below. Here is the problem -
  • Turn car on *with headlights*
  • Fuse between DC/DC converter and accessory battery blows
  • Accessory battery rapidly loses voltage - within 20min
  • Lights die, along with relay for motor controller
  • Car stops

Car runs fine without the headlights.

I assume that the additional load on the accessory battery pulls more current from the DC/DC converter, which blows the fuse. And the battery is damaged so it runs down quickly.

Per info on lead acid batteries and talking with ExpertPower tech support, max amperage charging lead acid batteries should not exceed 0.25C, or 5A for a 20Ah battery. The tech support guy said I probably damaged the battery, even if I had a 20A fuse between the DC/DC converter and the battery.

What is the solution, since most DC/DC converters are >30A, and I don't see a lot of huge Ah 12V accessory batteries in the EV forum?

Thanks!


Config -

Load -
  • 1.25A - with ignition on
  • 7-9A - ignition and headlights
  • 10+A - driving the car with headlights, vacuum pump, PS pump, etc.
Hi dh,

Assuming your load numbers are correct, indication would be that the 12V battery is drawing a charge current adding to the 10A load which totals more than the 20A fuse can handle.

Did the system used to work well? That answer should help you decide to increase fuse (and wire) rating or replace the auxiliary battery.

major
 

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Discussion Starter #3
I tried a 30A fuse, and that blew also.

Is it normal to have a 50A max output DC/DC converter connected into a 20Ah accessory battery? Do people normally fuse the connection between the DC/DC converter and the accessory battery? Thanks - Dave
 

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I tried a 30A fuse, and that blew also.

Is it normal to have a 50A max output DC/DC converter connected into a 20Ah accessory battery? Do people normally fuse the connection between the DC/DC converter and the accessory battery? Thanks - Dave
Did the system used to work well?

Your auxiliary battery should be sized to provide required energy to run necessary loads in the event of a DC/DC or main pack failure (or fuse blow). Necessary loads for you to decide and for how long. Like 4-way flashers should you be stranded roadside at night. Or to keep main contactor engaged.

I don't necessarily see anything wrong with your component choices. Fuses are required at the start of each branch circuit from a power source sized to protect the wiring, primarily, and maybe the load or source.

major
 

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If the DC-to-DC converter can put out 50 amps, and is trying to maintain a higher voltage than the battery current has, it's going to blow a 20A or even 30A fuse when you add some load (and thus add 8 amps or more of current flow). The obvious solution is to ensure that the wiring from converter to accessory battery can handle at least 50 amps, and fuse it for that. If you are concerned about that charge rate, the next obvious solution is a lower-capacity converter (since the current capacity is pointless). Can the converter be set to a lower maximum current?

If one of the purposes of the auxiliary battery is to handle current surges, it certainly isn't helping if it can only handle 6 amps and the converter can handle 50 amps. It seems like all it is doing for you is keeping stuff alive when the converter is shut down (the car is shut off).

The sealed lead-acid battery is the AGM type. I would expect it to be able to handle much more than 0.25C charge rate; they are routinely discharged at higher rates. Unfortunately, in this case the case is marked with rates, which calls for charge and discharge rates of no more than 6 amps (according to the manufacturer's web page for the ExpertPower EXP12200).

This size of battery is used in booster packs, which are used to start cars (which can briefly take a few hundred amps). 6 amps seems like a bizarrely low current limit.
 

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Discussion Starter #6
OK - so given that I don't know if the battery is damaged, and I cannot adjust output amperage on the converter, it seems that the best solution is to:

a) get a DC/DC converter with lower output amperage
b) find a battery with higher charge/discharge rates
c) fuse to max output of the converter

System failed the first time I turned the lights on (increasing accessory amperage), so I am not sure if this was a result of a damaged battery caused by exceeding battery spec for charge rate, or something else.

Any suggestions on a good converter with 20-30A max output? And a good battery that will stand 10-20A charge rate?

Thanks!
 

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I suggest you try a known good battery and make sure it is fully charged. Size doesn't really matter for this test.

Got any photos of the converter installation? Make, model and specs?

Are those current numbers in post#1 measured or calculated?

major
 

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Discussion Starter #8
Major -

I have ordered a new battery to see how that works.

The DC/DC converter is from Electric Car Parts Company - http://www.electriccarpartscompany....-Waterproof-to-3ft-1m-br-USA-Stock_p_749.html

I have attached a couple of files, including the label on the bottom of the unit, and a somewhat dark picture of the setup. This is in a Ford 1994 Ranger, with the converter mounted on front. Behind the converter you see the MR2 PS pump, and then the battery. The plastic box on the left contains all of the relays/fuses.

Thanks for your help - Dave
 

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