12 V battery running flat after 20 min of listening to the radio

First, the Prius 12V battery is smaller than a standard vehicle because the battery does not have to start the ICE. The ICE is turned over by MG1 (electric motor-generator) and the HV battery. So, the Prius battery is more susceptible to discharge than a typical car battery.

Second, the normal life expectancy of a battery is 4 to 5 years. Since yours is a 2007, and it has been fully discharged multiple times, I would highly recommend replacing the battery.

Third, if you want to listen to the radio for any length of time, keep the car in Ready mode. This way the HV battery is providing the 12V for the radio and to keep the 12V battery charged. When the HV battery SOC gets low the ICE will start and keep the HV battery recharged.

 

You will have the same sort of issue with any car and an old and failing battery.

 

Toyota Prius 12 Volt Auxiliary Battery with install kit for 2004 - 10

Ready is your friend.

 

1. Replace the 12V battery now. It is obvious the existing battery is near-dead.

2. When the 12V battery has discharged, do you charge it at least overnight? If not, the battery never has an opportunity to recover and is highly susceptible to further discharging as plate sulfation occurs and the battery capacity diminishes.

3. If the 12V battery is fully-charged then you should be able to listen to your radio while in ACC-ON mode (where no instrument panel warning lights appear) without difficulty for several hours.

4. If your habit is to leave the car in IG-ON mode (where most warning lights are on) then a very high current drain exists and the 12V battery will quickly discharge.

 

Assuming the OP has the stock radio, not modified the Sound System.

 

You are in Ireland? Can you get an Optima YTS 2.7J? It is a drop-in replacement deep cycle battery -- absolutely perfect for what you want to do. Only problem is that it is not available in the US, but it is in the UK! So your location does make a difference.

After running down the OEM 12V once or twice, it will never be the same.

 

I'm an engineer and I don't think users should have to put up with the 12V battery failing. It will fail eventually but it is easy for a car like a Prius to monitor the state of the 12V battery. It should also be checked in services and a sensible specification should exist for replacement. If Toyota think we need a 30AH battery then we need at least a 15AH battery to keep running. My car was sold to me with a 5AH battery that went flat went I used a compressor to pump up the tyres. The real problem is that 12V batteries need to be tested on a Prius. They need to do so little that they will be in a very bad state when the non technical driver can't turn on the computer.
I think they should be using the same care to look after the 12V battery as they do with the HV. A Prius 12V Battery does not need to be charged at 20Amps and neither does it need to be kept at 14V. This will just overcharge it. The float volts for a 12V battery is not 14V but more like 13.3 to 13.7.

 

Well, I'm not sure what facts you are talking about, but the following apply to the Prius 12V battery system:
The stock battery is a 38 A-Hr unit.
The "float voltage" is 13.8V when the Prius is in "ready". If it were lower the battery would never reach a reasonable state of charge unless driven for many hours at a time. If it were higher the battery life would be even shorter than it currently is.

If -your- battery had a lower capacity it has been damaged by over-discharging it and not fully charging it back up.
Like -any- lead acid battery, if discharged too much the battery will sulfate and its' life will be shortened.

The -real- solution for the Prius 12V battery would be to replace the lead-acid battery with a NiMH unit. Except it would be very expensive (and of course would require redesign of the charging system).

 

Lead acid batteries do not sulphate/ sulfate as soon as they are discharged. Sulphation takes time. If you recharge a completely flat battery within a few hours at a current of less than C10 it will recover happily. Sometimes the volys from a lead acid battery will increase as it is discharged as old chemicals are dissolved away. Sulphated batteries can also be charged with various methods that often work.

A solution for the Prius that would enable the 12V batteries to last a lot longer would be to limit the charge to 4 amps at 14V with a short boost to 15V. Followed by maintenance at the correct float voltage. An indication could also be supplied to the user of the current battery state of charge, and capacity and also its internal resistance. Not difficult. For the non technical a replacement time estimate could be given.

If you do flatten a 12v battery, do not fast charge it in the vehicle. Disconnect and charge it with a safe battery charger. It is not needed to supply high currents, and there is no reason for it to be deeply discharged. The car could even switch the load off before the battery gets so low that the computer cannot be booted. The high load will soon die anyway, why not switch it off before it discharges the battery below the computer boot on level.

There is still much energy available below 9V and that could be used with a cheap convertor to start up the computer or at least energise the HV relays to allow the 12V battery to be charged from the HV via the DC to DC convertor.

 

this may well become the 'rough guide' for battery testing, no instrument required

 

Just a thought, but surely the MFD would consume a lot of power from the 12v battery as it's an old cathode ray
design. Another reason to have the system switched to ready mode when listening to the radio.

 

Seamus the Optima battery is very good but in Europe is very expensive, UK price is £184. There are many other good batteries on the market here that are less than 1/3 of the price with longer guarantees and of higher capacity. Look at Varta and Bosch and Lucas they do 4 and 5 year guaranty batteries at around £40/60.

The Toyota battery is £90 with 1 year guaranty I believe the Optima has a 3 year.

John.

 

The MFD is not a cathode ray tube it's an LCD.

 

I already run a system for 12v battery recovery in case the interior light is left on etc. I use a small switch mode power supply from an old laptop computer fed via a DP switch from the HV battery. This feeds an LM317T connect as a current regulator set for 3amps and on to the 12v battery.